Chambers is about 7 spots down my private cheat sheet from Plaxico, but I think that in this case people are referring to the matchups. Chambers is up against Detroit, bottom feeders versus the pass, whereas Plaxico is up against the tough Patriots defense in what will probably be a low scoring ground game. The Pats DBs generally have great cover skills and good vertical leaps, so Plaxico's height might not be such an advantage vs. these guys.
On strength of schedule, I think I would start Chambers, but most weeks I would probably start my man Plaxico.
I'm not a big fan of starting anyone from the Steelers/Pats game if it can be avoided, other than the Defensive Teams themselves. Unless things get funky (sloppy passes, fumbles, long punt & kick returns), it will likely be a low scoring game that won't be helping anyone's fantasy numbers. The Place Kickers are probably the only offensive guys that might see good numbers (for their position).