Perhaps you've noticed that Version 1.9 is totally different to Version 2.0, which follws the MVC pattern ... I think your decision depends on your experience on ASP.NET: If you're familiar with classical ASP.NET, it's less effort to adapt a 1.90 installation - if you're new to both technics, I would suggest to take the 2.0 version and learn ASP.NET MVC 3 from the beginning, because MVC is much closer to the web and it'll be the way to go in web development on the microsoft plattform in future!
I know .NET and I've no problem to learn MVC. I use MVC paradigma on other CMS on PHP platform. We use classic form on .NET but I like to start with MVC.