The choice between WebForms and MVC really comes down to whether you like the Model-View-Controller approach. Personally, I prefer WebForms but am learning MVC (within Rails) just to broaden my experience and to gain the benefits of better testing. Either will do for your case as both are capable of scaling to large systems.
With respect to data, I would encourage you to think long and hard before selecting Linq in any form. Please see these previous discussions:
Doesn't LinqToSql Miss the Point?
Is LinqToSql Powerful Enough? Aren't fluent interfaces easy to build?
I would not recommend SubSonic as the documentation is just not there. You'll be pulling your hair out trying to figure out how it works. This is sad because, in many ways, it is a better model than Linq.
In the end, I have always chosen to go with a custom DAL wrapper around ADO.NET. First, I know what I am getting. Second, you really do have to know SQL in any event so why make life more difficult by trying to learn a second data access language? The drawbacks to Linq and SubSonic are manifest and the advantages are primarily theoretical (again, see the discussions at the links for more information).