I was having the same debate with myself on which one to buy, I narrowed my choices to the Sammy GTab, Toshiba Thrive and the Moto Xoom.
I ultimately ended up with the Xoom, the biggest reason being that it was the tab equivilent to the Nexus and my Epic and lack of or slow updates kind of turned me off to Sammy in regards to Android. I also like vanilla Honeycomb over cheesewhiz plus it felt like a more solid built device. Also the fact that Tiger Direct is selling refurbs for $399 swayed my descision a bit as well
The $200 difference is price between the Xoom and GTab for similar specs was kind of a no brainer.
So far a week with the Xoom and I love it, the thing is fast as hell, very stable and does pretty much most of what I need so far and I'm still learning more about it every day!