I was in a similar position... My workplace provides me with dealer lines for all the carriers, so I had the full spectrum of choice also.
I'll say upfront that I ruled out T-Mobile because of their terrible coverage in my area, so I never even looked at the HD7. I don't know how their coverage is in your area, but they are the smallest carrier, so I would look into that before considering a 2 year contract with them.
The choice between the Focus and Surround was tough at first, but I settled on the Surround and I'm extremely happy with it. I'm really not trying to spam this, but this is my comparison of the two http://forums.wpcentral.com/samsung-...-surround.html Edit: it's a couple posts down the page.
I'll say that if you want the best form factor, you'll wanna go with the Focus for sure. To me, the advantages to the Surround are worth the extra bulk, which really isn't a whole lot, and definitely not as much as you would expect.