I don't know enough about VMs to judge, but Hyperthreading sounds good for your use, even if it's already a quadcore, and the i7 sports "Intel® Virtualization Technology for Directed I/O (VT-d)", whatever that is. Those are the main i7 advantages I can see, the rest is of no concern as far as I can tell. The i5-3570K runs like a champ, and the price difference is quite big, at least in Finland.
I'm a stranger to mITX as well at this time, but here's an interesting-looking case: http://www.bitfenix.com/global/en/produ ... y#overview
. There's a lot of marketing and a lot of holes in the design (unfiltered side intake, restricted front intake), but it's a looker, and sports good compatibility with different fans on paper. The Lian Li mentioned by kuzzia I would think to be the smarter design though.