ASP.NET MVC2
Web Forms solved a problem several years ago but unfortunately things have changed significantly enough now:
- Introduction of agile development methodologies
- popularity of unit tests
- adherence to web standards*
- bloating of your pages (viewstate) in an attempt to make them stateful
These things have left Web Forms as slightly dated.
ASP.NET MVC was a blatant reaction to these changes and the competetion from Ruby on Rails. With MVC you will now find:
- Making your work unit testable easier
- The statelessness refreshing and lightweight
- The level of control increased
- Speed of development hugely improve
I'd suggest using it in conjunction with the Entity Framework 4 as well if you are looking for speed of development too.
If you need to learn one from scratch: learn ASP.NET MVC 2
*web standards have improved with ASP.NET 4