Nexus S on T-Mo takes advantage of HSPA +, but isn't fully compliant. yeah, it's confusing. In a HSPA+ 3G area, I get better speed on my Nexus S (a steady 5-7 Mbs down, 1-5 up, ping in the 60s) than i do with the G2X, which is always over 2 or 3 mbs down, but not as consistent.
In what T-mobile calles a 4G HSPA+ area (again, I'm as confused as you are here lol) I get a maximum 7.2 mb/s down on the Nexus S, but have seen 13 mbs down on the G2x. At Google I/O at the hotel, I was seeing 6.9 mbs down on the Nexus S, and 11.5 down on the G2X.
I'll be honest -- unless I'm tethering a computer, i see no difference in speed for browsing or streaming music/video once i get past ~3 mbs. Big difference on a PC while tethering though.
I hope I've throughly confused everyone lol. I no longer even care about the G's. I just call it normal, fast, and zomg.