Since you've said one of your reasons for migrating is to incorporate ajax, it would make sense to use Struts 2, since it has features that incorporate ajax.
It's important to understand, though, that you're not limited to the options you mentioned in your post. For example, with
EJB 3 you still need a persistence provider and there's no reason Hibernate couldn't be that provider. There's also no reason you couldn't incorporate Spring into a Struts 2 application if it has features you want.