why mvc is better?

Web forms doesn't follow a controller based pattern. Instead you can use a presenter pattern (MVP : model view presenter), which while similar to mvc, differs in that the presenter has direct access to ui elements whereas a controller does not.

In terms of seperation of concern, it's true that asp.net mvc in it's entirety is the presentation layer (at least in a well architeced application). However that seperation doesn't only apply to teams split across m, v and c. It's more about functional seperation and future maintenance. For example if new a project requirement is to create a different set of pages for mobile devices, that is easiy achieved by only creating new views. You don't had to touch the controller. Like wise, the model (assuming the data model) can be modified to use a different data source without changing controller or view code.

Personally I have never regretted switching to asp.net mvc. In addition to what everyone has said about testability, performance and seperation of concerns I want to add my two cents.

• Extensibility: Almost everything in asp.net is modifyable. If you don't like how something works, rip it out and roll your own. Don't like the model binder, trash it and create your own. Don't like the way controller actions work? great, build a custom controller factory. Even routing is fair game. Now contrast that with webforms, you can't easily change the way server controls work for example.
  
  
• The Model Binder: This is by far my favorite feature of asp.net mvc. I hate writing left to right assignment code, and I love the fact that if I structure my html correctly asp.net mvc will give me back a fully hydrated object.
  
  
• Routing : (this is a moot point because asp.net webforms 4 has it), but I love having the routing engine abstract away the url. I can easily change controllers without impacting the URL. I actually had to do this on a live site and it was an incredibly smooth process.
  
  
• More control of HTML: I like using JQuery and I'm comfortable with css. I want full control of my markup to be able to add advanced client side funtionality, I don't want to have pregenerated html that I can't control (this is better with webforms 4, but still)
  
  
• Simplicity: In my opinion asp.net mvc is easier to use compared to web forms. There is no page life cycle in the tradtional sense and you generally only have two action methods for GET and POST.
  
  
• Client Validation: I absolutely love the validation story in MVC3, and especially the fact that client validation is turned on by default. Simply adding a data annotation attribute to my entity and a little code to my controller gives me both client and server side validation? That's impressive.