Romney isn’t going to pick a VP that shines brighter than the boss
Everyone shines brighter than Romney and he knows it. Romney is going to select someone who will help him get to the White House. That's all that matters. Romney's no fool. He knows he's having a hard time closing the deal with conservatives and the candidate he picks will be expected to do just this (as well as deliver a battleground state).
At this point, I can't imagine him choosing anyone other than Marco Rubio.