Personally, I have decided to use both...
If it's a website (viewed online), I have decided to use ASP.NET MVC.
If it was an application (web application with a single purpose) I have decided to use web forms.
This decision is purely based on the case use and the solution you trying to deliver. If you are interested in good SEO and a faster website, MVC is much cleaner HTML and faster than web forms.
However if you after a complex functionality with a lot of filters, grids, postbacks on the same page and you are well experienced in Web Forms, just stick with it.