I've not played much with my Raspberry Pi (Model B, it was the Last run with only 256MB Ram.) I hadn't even got mine before they announced the 512MB upgrade would go in the next batch (i.e. another week or so I think, Lucky me...) I can say the following however based on my experiences.
-The Pi can do .NET, albeit not as nicely as the other platforms by default (i.e. no IDE yet that I'm aware of.) It will have the advantage of being more featured than many similar priced boards, especially as far as power is concerned. The Netduino (Which is nice for what it does,) is the closest priced item I'm aware of. Aside from built-in Analog pins and likely lower power consumption, it doesn't offer much in the way of advantages from a hardware standpoint. .NET Garbage collection is essentially nondeterministic, which may cause issues if your projects require extreme precision. This applies to many other languages, including Python, But you are at least able to work around it easier on the Pi by using another language. http://www.mono-project.com/Mono:ARM
-Loading apps feels SLOW off an SD card. Using a Micro Center 32GB SDHC Class 10 card, I found starting most applications (Even the most lightweight GUI browsers present) would take a few seconds to load. Some of this is probably psychological as it's not 'instant' like most of my machines are, and I can't hear a hard drive or see any sort of hourglass.
-Once loaded things seem to go well enough; Large webpages however seem to take a while to render.
-No RTC. You'll have to build your own. The unit does damned well at getting the time once it has internet however.
-The composite video does actually work halfway decently as far as video quality (Again, for what it is.) The problem is the default Window manager craps itself on the resolution, too many of the important dialogs run off the screen. I'm guessing I should try another but I haven't gotten around to it.
I suppose when you get down to it, the question is what you're looking for; .Net Gadgeteer devices will definitely have lower power consumption and easier access to Arduino Periphs. The Pi has more RAM and speed and would work better for more 'complex' tasks (Image processing comes to mind, especially given the graphics core.) The .Net devices have a nicer IDE, the Pi can give near immediate feedback (The GPIO pins are just regular files in /dev/.)