Is achieving high unit test code coverage a big part of the business requirement?
i.e is it a long project with plenty of future refactorings? Do requirements often change? Are you one of the lucky ones who does TDD?
Unit testing asp.net mvc will come a lot more natural. It's really true that the framework forces you to the pitfall of success.
However, with a lot (A LOT!) of discipline, ASP.Net MVC webform can also come close to the same code coverage with patterns like Model View Presenter of Presenter First pattern.

Is there going to be a lot of handwritten AJAX?
With the asp.net webform, writing ajax by hand can be of a pain, for e.g data isn't ready until specific page lifecycle event, etc. ASP.Net MVC doesn't have page lifecycle anymore. So writing ajax by hand will come more naturally.
The question now is, do you have enough reason to not use ASP.Net AJAX control toolkit. The answer might be yes if you're looking for the highest compliance for W3C Standard, effieciency, performance, etc.

Not being able to use user/web control?
Asp.net mvc do not have page lifecycle event anymore. The concept of ’˜web controls' is sort-of replaced by ’˜subcontrollers'. The down side is subcontrollers is still in mvc-contrib codeplex project.

Not having viewstate?
Pipeline code like this have to be written manually. Sticking to the true roots of html querystring, post, etc. This means more control over what's sent through, but also more manual work.

The good news is; viewstate is apparently coming back to Asp.net MVC. However I can't seem to find any other post about this anywhere else

Is having multiple forms on a page important?
Something you can't do on asp.net webform, but easily achieved by using a bare to the metal framework like Asp.net MVC

Are you just sick and tired of the page/control lifecycle intricacies?
Well, I am.

Having said that, sometime choosing a platform is about productivity. Using Asp.net Webform does makes sense in many cases. Despite its shortcoming, when developed properly Asp.Net can be not only a highly productive tool but also produce fantastic results. You just have to be cautious of the pitfalls.