Helu is averaging 127 yards per game in the four games he was given the majority of the carries.
There's no way you can argue that Helu isn't lightyears ahead of Lucky in actually running the ball (and he's only a sophomore...Lucky is a senior). Just watching them in games it is pretty easy to see. You don't see Helu go down too often at first contact; you do see Lucky go down alot at first contact. Helu makes people miss and tries to find a hole; Lucky usually just ends up running into the back of one of his offensive linemen trying to eek out some yards (much like Castille does). Helu has better vision and just seems more athletic than Lucky.
Now, Helu does have some work to do on his receiving game and to a larger extent his blocking ability.
Who would I choose to be my primary running back? Helu. Does it matter what I think? No, coaches will start Lucky if he is able to go.