I have a 2011 335i Coupe Loaner car (Auto) while my 135i is in for some recalls. Some quick comments about N54 vs. N55 that I've experienced.
The N55 feels more responsive in the lower RPM range (below 3000 rpm) in terms of initial response/thrust at the first second you are on throttle (after not being on it at all, aka, no boost). In terms of overall thrust however, the N54 feels stronger (once turbos are spooled) from around 2500-5700 rpm. Anything above 5700 rpm, the n55 feel ways stronger and pulls in a more linear fashion than the n55 (obviously due to the differences in turbo setup where the n54 more quickly feels like boost is being tapered).
Just my experience and opinion FWIW.