wierd, you should be getting same results. I had a nexus one, had high 3.0mbs to low 4mb. with the nexus s i get about .4-.8mbs higher, have hit 5mbs alot.
thing is, i believe with future radio updates this thing will smooth out in regards to loyalty in staying in that arena, i have noticed some dropped out 3g coverage on occasion and no refresh after coming out of a subway..which was just wierd.
**just think of it this way, the nexus one got massive improvements after 6months of it existing and then perfected in the last 2-3 months. the nexus s should have the same principles.**