That's basically an impossible question to answer because it relies on an unknown hypothetical. If, for example, Bailey were the main girlfriend, would she still be like the Bailey we know? Well, no, since her character is reliant on her history with Rainn (which she wouldn't have if the MC and her had already been together pre-WVM) and her already being a cheerleader and all that.
And the same thing the other way around, Jamie wouldn't really be Jamie if she wasn't already the MC's girlfriend. Her character couldn't be to be the pimpqueen because, well, she wouldn't be in that situation yet. So she would have to be a different character altogether.
The only question you can ask is if we're talking strictly about the model, what she looks like, would Jamie be an attractive character that people (based solely on that metric) would like to pursue as a love interest even if she weren't already the established girlfriend? Well, I certainly think so, I think she's pretty hot.