I really like getting XP based on completing objectives. Not only that makes more sense, it also avoids things like scout mech pilots getting a lot less XP than the guy piloting the full SML mech that always gets to incapacitate the enemy.
Now, ace pilots could get some different symbols on their pictures/a new bio entry do signify their ability to take down enemy mechs. Like WW2 pilots - having little mechs in their big picture showing off how many of those were killed (maybe one for each weight category)