I thought I'd offer my thoughts on each type of format and some similarities and differences between the 2. I should mention I played limit hold'em the 1st 6 years of my career and nl the past 6-7 years of my career but in no way am I saying I'm an expert but I do consider myself proficient in both (although my limit hold'em game probably has been passed by the new generation).
I'll talk about limit hold'em 1st. It's mostly a game of maximizing bets when you have the best (best hand or more specifically +ev spots in any situation) and minimizing bets when you are behind. When you have top pair top kicker you usually have the best hand and want to be betting and raising at just about every opportunity. Also you can win at a pretty good rate just playing tight and straightforward up to midstakes (live). I'm kind of out of the loop when it comes to limit hold'em games online nowadays but figure they're probably tougher than they were when I was playing full time.
It is a simpler game than nl. You can basically memorize a chart of hands to play in certain positions. I think David Sklansky had some good limit hold'em books that included charts. Obviously full ring is going to be simpler than 6max or hu (the same applies to nl for the most part) because you can play straightforward more and actually just play based on the absolute strength on your own hand sometimes whereas shorthanded (and in nl) you have to be aware of the range of hands of your opponents (so relative strength of your hand).
Also it is a game where you or your opponents are correct in calling a high % of time at river because the pot odds are so great since it only costs 1 big bet to call (to win a 5+ big bets pot usually). Also it helps to know the mathematics of poker (obvious statement I know). There is an interesting scenario where you get dealt any pp and are supposed to raise preflop if you see 8+ players limp in front of you whether in late position (assume 1 or both blinds call if lp) or in the blinds. This even includes raising 22. The reason being that it's about 7.5-1 to flop a set so therefore you're getting an overlay on any additional money put in because you can expect everyone to call 1 more small bet preflop. The times you wouldn't do it is if you're suspicious the 1st limper limped w/ the intent of 3betting but most don't do that. Also I'm just throwing out any postflop scenarios because it would complicate things ...eg. flopping underset, losing to straights +, but it prob. evens out or shows a profit overall when players are tied to their draws or top pair type hands because of the bloated pots (plus you can win unimproved sometimes).
I remember playing $100-$200 a few years ago and a very good limit hold'em player checked 77 from the bb after 8 people had called preflop for 1 bet (sometimes the games were good even at those stakes back then). Anyway he flopped a set and won a big pot but should've been a bigger pot for the reasons I mentioned.
As for winrates, it was assumed when I was playing that the very good players won at a rate of 1 big bet/hr (live) so at $20-$40, about $40/hr. I'm not too familiar w/ winrates online even though I played a lot during the PartyPoker days (from $10-$20 to $100-$200, $30-$60 being the main game for me).
Now I'm going to talk about nl which is a much more familiar game to me now. I have to admit I made a ton of mistakes early on in my nl career, mistakes that most nl players would not make if they just started playing nl from the get go. It was due to the fact that I was playing nl a lot like limit early on because that's pretty much how I knew to play hold'em. Top pair good kicker? No problem, stack off on the flop. Shove turn or river as a bluff? Umm, nah because I could lose my whole stack whereas in limit I would lose just 1 big bet.
I was able to win from the get go because I've always had a good feel for any form of poker and played my A game consistently (not tilting). I've since found out that nl is a much more pure form of poker in that you are playing your opponents more than your actual hands (whereas in limit you are relying on your and your opponents' hands more). Also nl is a game where you can win pots by skillfully executing lines that are credible, inducing opponents to bluff or shove w/ worse, threatening opponents for their entire stacks, among other things that doesn't apply to limit as much.
There is a reason why the worst players in nl games went broke much earlier than the worst players in limit games. It took constant pounding (of many hands) to break a bad limit hold'em player but it could take just 1 hand to break a bad nl player.
Also the range of hands you play in each game differs. In limit hands that flop top pairs w/ good kickers trump (along w/ mid to big pps) such as KJ, KQ, AJ, etc because you can bet bet bet whenever you flop a big pair (obv. slow down on certain board textures and vs certain opponents). There are a couple of reasons for this...you can't lose too much if you're behind and a lot of players play any 2 broadways so you'll have them outkicked enough times. The flip side is you don't want to play small cards, even suited, like 65s unless you get 4+ players going into the flop. It's a volume hand meaning you need lots of players in to make it profitable to play. Ideally you're hoping to make a straight or a flush and it's hard to make those. Also when you do make one you can only win a couple of big bets.
In nl it's sort of backwards for the hands I mentioned. You generally don't want to play QJo, KJo, ATo, esp. when there is a raise because you don't want to come in 2nd place (kicker problems) because it's hard to fold when you make top pair and will end up losing too much vs. winning enough to make up for the losses. You'll probably lose at least 2 streets of betting if you're behind and 2 streets of betting in nl is a whole lot more than 2 streets of betting in limit. Also suited connectors, even the smaller ones like 45s have a lot more value because the implied odds are huge (in limit you may win 2-3 big bets, in nl you may win opponent's entire stack) so you can play them frequently.
As for bankrolls for each form of poker, it was generally thought that you needed 300 big bets for whatever stakes you were playing for limit hold'em and 30 buyins for nl (assuming you're a winning player). Obviously it really depends on your skill level relative to your opponents'. If it is narrow then you might need more of a bankroll and if the gap is wide then you could use less. Now w/ the games being tough as ever today I would probably say 1.5-2x the aforementioned bankrolls would be advised.
Now since I live in Vegas I'll just say the best place to play limit hold'em is Bellagio where they spread $4-$8 up to $60-$120 on a daily basis. Anything higher is dependent on a few things. There are lots of $15-$30 and $30-$60 limit games and for the most part they're still beatable if you have a solid grasp of the game and don't tilt much (and have good br management..again stating the obvious). They used to have a $100-$200 game daily (which I frequented back in the days) but it kinda died. There are $10-$20 and smaller games around town as well. As for nl I still like the Bellagio but since nl is king right now there are games everywhere around town. The highest games that run daily outside of Bellagio is prob. $5-$10 (uncapped) at Venetian and the Wynn. Bellagio has several different stakes and runs $10-$20 nl daily which is a pretty good game usually. They'll get a $25-$50 game sometimes (and much bigger in Bobby's room).
I'm sure I didn't cover everything when it comes to limit vs nl but I hope you can see some of the differences in the 2 variations of the games. Both can be fun but for the most part, nl is tougher to get a grasp of but probably more fun because there are more options available (my thoughts anyway). Also if you're starting out I'd recommend learning nl 1st because there are many more games available than limit and also just about every tournament is a nl tournament so you'd like to have the option of playing cash games or tournaments when you want. I am a believer in learning as many forms of poker as you can because it's nice to have a variety of games to choose from day to day (plus there might be softer games in one form of poker than the other form) and this inc. limit, nl, mix games, plo, and whatever other forms of poker is out there today.
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