We played 2gap last night.
There aren't really big differences between the 34 and 43. We are going to use the same personell. We are going to have a front 7 player across from each OL spot (meaning weak OLB is free if there is no TE, buit a play coming that way must have a pulling G or FB assigned to him)
In the 34 or 43 we rush 4 if we don't blitz.
The advantage in the passing game to the 34 is that one of the LBs will be the 4th rusher, but no one knows which (although it pretty likely one of the OLBs) and in the 43 there is no disguise.
In the run game the argument is the NT being in the middle of the field can control much of the run game, while in the 43 the MLB is tough to get a clean block on.
Really there are differences, but they are minor, and it really comes down to which alignment is a better fit for your personell, particularly the LBs, because their roles change more than the DL who simply 2gap one guy or another.