Orchard runs on ASP.NET MVC - it was designed like that from the very beginning. Building apps with it is much like building ordinary ASP.NET MVC 3 application, so the learning curve is not that steep. Also Orchard delivers an extensible component framework to ease building MVC apps, so I guess it's much more than a CMS. And at it's core it uses pure MVC approach (fully controllable and no trash markup attached) to render final output.
Don't know much about Umbraco, though. Tried that some time ago and at first sight it just looked overcomplicated, so I searched for other solution and sticked to Orchard as it appeared pure and simple. It was over a year ago, so at that time Orchard was in pretty early stage (<0.5), not ready for production use, though. But lots have changed since then and now it's a full-featured platform with large community behind it. Not to add that Orchard is free, open-source and has a strong Microsoft support (financial and personal). The team working on it is all ASP.NET guys, so I guess it can't be better:)
Btw - the newest Orchard release - 1.1 - will be officially announced on upcoming MIX'11.
I'm currently building two large commercial applications based on Orchard and I'm just very happy with it.