If you're a customer in good standing, Verizon will generally give you the unlock code, so you could use the Droid 2 Global with another provider's SIM card. Just call customer service and ask them about it.
The Nexus S is a really lovely phone, and I own one myself. You're best off going with T-Mobile if you get that phone though, because it lacks AT&T's 3G bands. Android is absolutely not great with 2G/EDGE service because it does a lot of updates over the data network. These go very quickly on 3G and crawl on EDGE.