Every year we get back to this debate. Should teams go for the 4-3 defense or the 3-4 defense?
To answer that, we have to back up about a decade when only a few teams ran their defenses with a base 3-4. In those times, teams were picking players from a better pool because no one else wanted 3-4 linemen — or at least not as badly.
Now, times have changed. About half the league runs a 4-3 defense, and the other half runs a 3-4. Yes, there are a few hybrid defenses out there, but eventually, hybrids tend to fall on one side of the fence or the other.
The advantage of running a 4-3 defense are relatively obvious. It allows you to put more big men on the line as well as giving you a stable base to play defense from. Without great creativity, the 4-3 defense has the ability to succeed on pure talent, meaning that best units in the league don't have to rely on exotic looks to beat the offense.
The 3-4, on the other hand, allows for a huge amount of creativity, but that comes at a cost. Defensive linemen are largely restricted to holding the point of attack while linebackers shoot through gaps, making the big plays. This also means that the 3-4 defense will be far less predictable.
At its finest, the 4-3 defense better allows all eleven defensive players to make positive contributions. Instead of sending a linebacker to go after the quarterback, either defensive end in a 4-3 system can beat the tackle and get in on the quarterback. Because there's two DTs instead of one, it's far easier for interior defensive linemen to put pressure on the quarterback as well.
In other words, the 4-3 defense is a great, stable alignment that allows players to play fast, but it only works if those players are of high quality. In a poor 4-3 alignment, no pressure is put on the quarterback, and the defense can't hold together long enough to prevent big plays.
That's where the 3-4 defense comes in. It allows the defense to hide some of its problems by confusing the offense into making mistakes. With the 3-4, it all starts up front with three big linemen. If they're not strong, the rest of the defense will suffer.
Ultimately, both styles of defense can be very successful. 3-4 teams employing top defenders are terrifying, and 4-3 pass rushers demand top dollar in the NFL. There's no "right" or "wrong" answer to which defense to pick, so long as a team can put the right type of players on the field to fit the style.