The defensive ends in a 4-3 defense should be big enough to take on an offensive tackle one-on-one, but athletic enough to get around the tackle and get to the quarterback. The defensive ends in a 3-4 defense, though, are similar in size and responsibility to the defensive tackles in the 4-3. They must be stout against the run and should take on multiple blockers to help out the linebackers. Pass rushing from a 3-4 defensive end is something of a bonus.
The outside linebackers in a 4-3 defense can get away with being a little undersized because their primary responsibility is stopping the run and dropping into coverage. The outside linebackers in a 3-4 defense, though, are the playmakers. They are often referred to as "tweeners" because their size and athleticism fall somewhere between that of the outside linebacker and the defensive end in a 4-3 defense. The 3-4 outside linebackers are primarily pass rushers, so they must be big enough to take on the tackle, but athletic and nimble enough to drop into pass coverage, as well.
While it is usually more difficult to find the personnel for a 3-4 defense - since it requires a mammoth presence inside at nose tackle and exceptional athletes at outside linebacker - it is often preferred by defensive coaches who like the flexibility and unpredictability it creates in rushing the passer. The defense is never certain which linebackers will be rushing the quarterback and which will be dropping into coverage. The 4-3 is more versatile in terms of utilizing the strengths and characteristics of a group of players, and therefore the more common of the two.