Whats the difference between defensive ends in a 3-4 vs in a 4-3 defensive scheme?

The difference lies more in the scheme than the front. Defensive ends can have the same responsibility in both fronts. Most 3-4 teams have their linemen 2 gap and in a 4-3 they'll just 1 gap. But it varies and you can do either or. A 4-3 under front can essentially be 3-4 front with the responsibilities of either.

DE's in a 3-4 scheme usually are usually bigger, don't rush the passer and are great against the run and setting the edge. OLB's are used more as pass rushers in that scheme. In a 4-3, DE's are often times the same size as OLB's and primarily rush the passer.

3 down linemen vs 4 in a 4-3 scheme. In a 3-4 you usually have a big NT that can play 2 or 3 gaps, another DE that can do the same so the WLB is free to rush the passer, it's better for disguising blitzes. A Safety can also lineup as a LB and perform the duties of one. A 4-3 is kind of predictable, the DE's rush the passer and the LB's either drop back in coverage or play the run.