What is the differance, personell wise, between the 3-4 and the 4-3 defense

The 3:

2 DEs that weigh between 290-310 lbs; guys that would usually play DT in a 4-3.
The Prototype: Richard Seymour.
The draft prospects: Glenn Dorsey, Sedrick Ellis

1 NT that usually weighs 330+ lbs. Good ones that can hold up for the whole game are a commodity; that's why guys like Ted Washington and Keith Traylor had such long careers.
The Prototype: Jamal Williams
The Prospects: These guys are a rarity. None stick out this year unless someone gains some weight.

The 4:

2 ILB. Same build as 4-3 MLBs. Must be versatile enough to play the run and drop back into coverage. Ironically, I think Simoneau could be a pretty good TED LB in a 3-4. The TED typically drops back and plays the pass over the middle and cleans up whatever is left behind by the MIKE in running situations.
The Prototype: Donnie Edwards (who's now in a 4-3 in KC)
The Prospects: James Lauriaitis, Jonathan Goff, J Leman, Brandon Miller

2 OLB. THIS is where it gets tricky. You're looking for guys who are 250-275 lbs. You need pass-rushing specialists bigger than 4-3 OLBs yet smaller than 4-3 DEs. These guys are often referred to as "hybrid" linemen in draft circles. Typically they play DE in college, but are considered to small to play DE in most schemes in the NFL.
The Prototype: Shawne Merriman.
The prospects: Vernon Gholston, Lawrence Jackson, Shawn Crable (who also makes a great prospect in a 4-3.)