If Leary doesn't play, I'd assume that Bernadeau starts, just because he's more experienced. Not that I wouldn't love to see Collins in there, because I would. I just think it would be a huge mistake if Romo got injured because Collins made a rookie mistake on picking up a rusher that Bernadeau would've got.
If Collins starts, it's because the coaching staff feels comfortable with him in there and feel that he gives them more upside than Bernadeau. If that happens and Collins has a great game, it wouldn't surprise me if Leary doesn't get his spot back.