nexus s, it has a bigger/nicer screen, faster processor, android 2.3 already loaded. It's the better phone but the g2 does have a physical keyboard which could be a deal breaker for some. Also the g2 doesn't run pure android as the nexus s does
The Nexus S has no external memory option (no microSD card slot). It does have a lot of internal memory, and this may placate some people, but I personally like having the option of adding an extra 16 or 32 GB of music files to my Nexus One.
Frankly, if I were using T-Mobile and had to select, I'd get the new MyTouch 4G. Check it out.