Now that it's RTMed and now that there are some very good resources on it I would say ASP.net MVC would be my strong preference, but it's not cut and dried.
Web forms hasn't gone though, it's still there, it's still supported and I've worked on several major sites and used Web Forms very successfully, so if there were other external factors such as a customer preference, or perhaps a team that had solid Web Forms experience then I'd still be happy to work with Web Forms. That said I have already worked on one project with MVC (while it was still in preview), and I much prefer it - my reasons are similar to those given above so I won't repeat them all. I will say that if testability isn't the best reason it's certainly in the top one:).
