I'd go with the dual Xeons.
Today the 8x1.8Ghz will be slower than the 4+HTx3Ghz i7 for purely computation tasks - BUT the memory bandwidth for the dual Xeons will be higher and in the future you can easily swap out the Xeons for ones with HT or higher frequencies when they drop in price, this may coincide with an overall increase in system load too.
The other thing to consider is that with dual Xeons you can do 24GB via 6 x cheaper 4GB DIMMs, to do the same for an i7 will require 3 x much more expensive 8GB DIMMs, plus you'll have lots of spare slots open on the xeons if you want to upgrade in the future, you'll have less on the i7.
Edit - do you HAVE to use Hyper-V? ESXi is equally free but you're likely to get a lot more VMs into the same memory.