Trying to make the Android transition.

It's been pretty easy to move my data and stuff over to Google services, all things considered. I'm not like some mega power user anyway. Here is the rub, I'm locked in a contract and my carrier (Sprint) only offers one Nexus device (Galaxy S). I really really really really really don't want a skin or modified version of Android.

Also, I'm broke. Ideally, I could use use some suggestions that are low cost. Is a second hand Nexus S a decent move from an iPhone 4S? Is it worth dumping Sprint altogether for a T-Mobile Nexus 4?