If you're using MVC 3 you'll still be running on ASP.NET.
MVC is a framework that leverages ASP.NET, so you get the benefits of the established platform with (IMO) a better view engine and better separation of concerns.
Auth/Auth is very easy in ASP.NET.[
^], and therefore in MVC as well.
MVC 3 also provides the Razor view engine, which is slick. If you're not opposed to learning a new syntax and you're comfortable with the MVC design pattern, I would recommend MVC 3.
If you prefer event-driven programming and want "free" viewstate management, go with ASP.NET sans MVC.
For the scope of the project and the tooling available, I'd choose MVC 3.
Cheers.