Well... depends on how you define stronger. Starkiller is probably a deadlier guy in combat, and his telekinetic force powers are impressive indeed. But in general he is trained for that purpose to start with.
Luke seems to be a bit more well rounded though. But I do think that he is less deadly in combat, even though combat does not mean everything.
Even with the toned down level of force from the game, Starkiller is still quite a powerful one.