definitely the new way, but not because it is easier for the sake of being easier. the challenges forced you to play a certain way, often in an unfun way. kill x all-stars? oh wait, it is 6 on 6 and there aren't any all-stars, for example. destroy 3 teleporters with chomper? oh, you cant get to it with the turrets and engineer camping it? there isn't an engineer building teleporters?
anyway, you get the point. the xp system may not be very innovative, original or difficult, but it beats those challenges hands down. It can be fairly quick to level, but with 5k+ levels, I don't think you can make it too slow either.