Cutscene badassery vs actual gameplay is not an accurate comparison of strength. Nor are power up stories, for that matter. The WoL had to lose to Zenos the first time otherwise there'd be no buildup to the rematch. Even if the WoL was stronger they still would have lost for "reasons" simply because the script called for it.
We have no real way of knowing who would win in an actual, fair fight because the answer is always "whoever the writers pick."