As to who should win, Ronda Rousey is the obvious answer. As to who would win, I think Rousey again comes out with it.
A couple of things work in Rousey's favor. Rousey is a bigger star than Nikki Bella has ever been or ever thought about being. Also, let's be honest, she pulls in more money for WWE than Nikki has, does or ever will. Nikki is also someone that's more reminiscent of the "Divas" era and is more of the past when it comes to the "Women's Evolution." If WWE is intent on moving forward with women's wrestling, then losing to Nikki does nobody any favors. It also doesn't hurt that Rousey is obviously quite close to Triple H and Stephanie and has some pretty significant neck issues that she's never recovered from. I mean, if some of what's shown on Total Bellas is legit, then it's hard for Nikki to take any real significant bump without aggravating her neck issues. Even though she's had surgery on it, it could be one of those issues that never really, 100% goes away and, from what I understand, that's not at all uncommon for neck and back issues. She takes one good hard slam to the mat via Ronda Rousey's judo throw and she could be in a world of hurt.
One final thing potentially working against her, especially when you take everything else into consideration, is that she's no longer John Cena's squeeze. Again, there's any legitimacy to Total Divas and if various reports are accurate, Nikki feels her biological clock ticking and wants to start a family, and Cena does no want kids whatsoever.