It looks like a 3-4 outside linebacker is more of a standup DE with some additional duties that include playing in space. This position appears to be more of a 'point of attack' style position. A 4-3 linebacker seems to play more off the line and is more of a 'play in space' kind of player that is more likely to pick up TE, backs for coverage and have more outside contain responsibility. I rarely see 4-3 linebackers get many sacks.
Is it common for a team that uses a 3-4 to have one of their outside linebackers be more of a blitzing linebacker and the other be a play in space person? Then the inside linebacker to the side of the blitzer, shift out wide. So in a base defense a 3-4 would be very similiar to a 4-3, but with 1 DE standing up. This is how I think the cowboys used Demarcus Ware. So now that he is switching to a DE in a 4-3, its very similiar to his previous position.