I learned something interesting from a friend of mine who knows about such things - Managed DirectX really isn't ready for prime time.  In fact, it's more of a gimmick, and nobody at the XNA group is championing it, which means it's probably not a viable development platform.

So if I'm going to do my Tank game, I'll probably have to use C++ and something like Torque.  Not that that's bad, but there you have it.