that's dependent on the problem at hand. Are you building a site that has a lot of moving parts on the same page, that needs to communicate nuanced state changes across components? If so, react/flux is a godsend, since the system will be easier to maintain and customize.
Is your site made up of many different views that don't change much once loaded? Then angular would be a fine tool for the problem. You would likely move much faster initially, since Angular handles a lot of problems with data binding and routing for you.
And finally, are you sure you need either of these frameworks? If you want your front end to consume JSON but it isn't particularly dynamic, something like backbone will probably be good enough. Hell, server-rendered templates do the job in a lot of cases. No reason to solve a problem that doesn't exist.