Still not a resolved issue I see. I'll try again...
If you want a fully fleged website with nice looking URLs and MVC based architecture, ASP.NET MVC is obviously the way to go. Now, I totally understand this is not what you're asking - and to be honest, your question is slightly vague, so I might not get it this time around either.
WebHTTP in .NET 4 is a slight simplification of the WCF services that were introduced back in .NET 3, and it follows the trends in web development today. WebHTTP has a really extensive support for customizing your URLs, controlling the response that would be tough - or basically just alot of work - to achieve with regular WCF and also in MVC for that matter.
Picture buying a pizzaslicer and a pair of scissors. You can cut pizza with both, but the slicer will undoubtably be more efficient. You can also cut paper with both, but the best result will be with the scissors.
You can achieve the same things with MVC and WebHTTP, but for instance creating a View is just alot more straight forward with MVC because that's part of its main function (it's actually in the abbreviation). Varying the responsetype from one single method, on the other hand, is a trivial task in WebHTTP whereas in MVC it requires more tinkering. The same goes for making a RESTful service. WebHTTP was made for stuff like that - MVC was not.
Bottom line - you can achieve pretty much the same things in both, but they are tailored for different needs.
If this wasn't the "right" answer either, perhaps you could provide some background for your question?