As most people will say I am sure, it very much depends what you want to learn and the style of development you wish to take on. If you want a lot of control of your page markup and intend to use a lot of AJAX and CSS, then MVC is a better approach, the biggest trade off is you lose the Microsoft controls such as the Gridview and other drag and drop components. MVC is very powerful and extensible framework and learning it will also allow you learn the MVC pattern (which is useful if you ever diversify your skills later on to other development platforms).
However if you are creating lots internal web applications that need to be delivered quickly, then Webforms is certainly a better way to go, it's well documented and mature and the controls allow you build powerful functionality for free. Although you will get basic templates to manipulate data in MVC, it isn't anywhere near as fast a building applications in Webforms.
My personal recommendation is to learn MVC (and it's pattern) as it does not have a big reliance proprietary Microsoft components and makes you think about your application on the front and it's interaction with the back end.
If you want a good example of a big MVC site... you're on it.. stackoverflow run on ASP.NET MVC.
Hope this helps, it is a very general opinion and there is a lot more to both arguments. However from personal experience I think MVC will teach you a lot more reusable standard web technologies in the long run.
Ciao
Jon