Both can be good schemes if used correctly. The 3-4 is currently very trendy, and it has been successful with a good amount of lbers. Note that the 0 gap nose tackle is starting to become scarce as other teams go to the scheme.
About 5-6 years ago, the big thing in the NFL was the TB/Monte Kiffin/Tony Dungy cover 2 - a 4-3 form based on getting QB pressure with your 4 down lineman and playing a disciplined zone in your secondary. For a while, if you could just find some good lineman, you could play this scheme - but so many went to it that ends became harder to find (note - they will always be hard to find - a true 3point pass rusher is a very difficult commodity to find - very, very effective, but tough to find).
Basically - they are all schemes, and only as good as the personnell you can plug into. Neither one is really better than the other if it can't be run effectively. For instance - the Ravens have always had a good defense, and have gone back and forth over the last 5 years - they started (and won a SB) in a 4-3 scheme that was very successful, have for several years been a 3-4, and last year reverted back to a 4-3.
Also, outside of scheme, it is important to note that there are 2 general defensive mentalities - 1) beating an offense by masking what you area throwing at them and 2) physically being better and more disciplined.
An example of 1 - beating a team by masking your defense: This style is favored by defensive coaches such as Buddy Ryan (the 46 defense), Jim Johnson, Gregg Williams, etc. They show you many, many different fronts - the goal is to confuse the QB and the offense as to what you are doing. For instance - your line is spread wide, your linebackers are showing zone, and then, all of a sudden at the snap, you get a gap blitz attack from your weak and middle lber - the QB never saw it coming and takes you by surprise. Often times lower quality DBs excel in this scheme because the idea is to get pressure - see teams like Philadelphia, where Jim is the master of the blitz package.
Example 2 - the 'Beat you Straight Up' style coach. Coaches like this are Bill Parcells, Tony Dungy, Landry. Basically - these defenses don't try to do smoke and mirror - they try to plug talented, start defenders into the right place and make sure they know their assignments. You rarely see complex blitzes - maybe a safety blitz, a zone blitz, perhaps the MLB will hit the 2 gap. But the idea here is discipline - get pressure with your front 4 or 5 (depending on how many you rush), make sure everyone stays with their assignments, etc. If you have good players, this works really well without having to be too complex. Note that Parcells and Dungy is an example of this - I think that this is one of the reasons Zimmer is here - Zimmer used to be a big fan of the cover 2, which would fall under this category - you rarely blitz in that scheme and rely on QB pressure from your front 4. Parcells had a press conference last year discussing these two types of defensive mentality and expressed that he prefers this style. This kind of explains why you have never really seen a complex blitz package past a zone blitz in our defense.
Anyway - the point to all this is that there really is no 'right' way to do it - the key is good players and, perhaps even more importantly, good coaching that knows how to use these players.
