The rifle you speak of is already out, it's the XCR-L. It's available in 5.56, 6.8SPC, 7.62x39, and hopefully 6.5 Grendel Very soon.
The XCR-M will be made in .308/7.62x51. I am really hoping they will do the same thing and offer caliber conversion options as well. A mag full of 10 .338 rounds would be disgustingly awesome.
With the way delays in a small company affect things, I wouldn't expect to see the XCR-M till July next year.
As far as deciding on 7mm Rem, or 300 Win Mag... I'd just hope the rifle makes it out before 2010, then hope they do conversions. I suspect that thing won't be nearly as easy to convert around due to ammo length and mag issue.