Nowadays I'd say it's not incredibly hard to find people proficient with .NET or Java, or people willing to learn, and with the advantage that a managed language is much easier to develop in than one like C++. In the long run you'll probably regret not going for managed.
Veteran, in six months there was a semi-working geoscape, in 11 months the geoscape was almost fully featured, only last year progress slowed down again.
Note, however, that these numbers are considering that the programming team was composed of one single programmer; i'd say it's pretty impressive progress rate considering it's a one man team, and one who isn't a kid with all the time in the world.
I'd suggest if you go the XNA route you download the XNA branch from Project Xenocide's SVN and either take some pointers or find a use for PX code for your own progress (open source, after all, no harm in asking for permission of course).