4-3.... 3-4..... Why care?
There are many differences between these 2 base D schemes, but what people fail to consider is why one might be favored over the other. The main reason is personnel, and it is not just "who" happens to be on your team, it is how difficult or easy it may be to find the talent to effectively implement the schemes.
The 3-4 two gap (bend but don't break) is the easiest to find talent for. You need DL to occupy blockers so the LB's can make the tackle, hopefully, within 3 yards of the line of scrimmage.
The 3-4 one gap is an attacking style and requires DL to shoot the gap, get into the backfield, and disrupt the play so the LB's can clean up. So, you do not necessarily need a DL who can stand his ground, because a quick DL can fill this responsibility. The skill level of the pass rush LB makes or breaks this D.
In both 3-4 D's, the 4 LB's are the most important part. You can survive with decent, not great DL. The 3-4 schemes are so popular because:
To run a top 10 NFL D out of a base 4-3 you need 4 exceptional guys on the DL. Guys who can rush the passer, disrupt plays, and stand their ground. These guys are really, really hard to find. But when you get 4 of them on a line together, watch out. Put 4 guys who are difficult to block 1 on 1 on the same line and you suddenly have a D that opposing teams can not scheme to beat.
So, why doesn't everyone put together a great D line and play a 4-3? First, you've got to have the patience to use first round picks in successive years to acquire the players. Second, once they've achieved success, it becomes costly to sign and keep them all. It's much easier to try to find one really good DL, one really good OLB, and just fill the other spots in a 3-4.
Personally, I am hopeful that Karl Dunbar's presence means we are going to see a lot more 4-3. If you draft correctly, there is no scheme an opposing O can run to beat 4 beasts on the DL. Remember how the Vikings entire D was Jared Allen, Ray Edwards, Pat Williams, and Kevin Williams. Now imagine if the Williams boys were more in the mold of John Randle - not just space eaters - In a word, unstoppable. I only hope that the Jets past 3 drafts are an indication that they are attempting to create such a DL.