I would choose MVC simply because it's designed to be testable and mock'able. That would be the major factor in my decision.
WebForms are much more difficult to Unit Test because they're rooted in several concrete classes that are difficult, it at all possible, to Mock. These include HttpContext, HttpResponse, HttpRequest and HttpCookie.
MVC is designed to be testable and it's API greatly facilitates doing so.
Good article on the testability of MVC: http://dotnetslackers.com/articles/aspnet/ASPNETMVCFrameworkPart2.aspx
