5.1 will have 5 speakers + subwoofer. 9.1 will have 9 speakers + subwoofer.
every single movie encoded with surround sound will work perfect on 5.1 however you might find less content recorded for 7.1 and even less for 9.1. in many cases you might have to fake the extra channels in with a dolby digital sound mode. so while the 7.1 or 9.1 might be advantageous in some movies in others not so much.
a word of advice: home theater in a box (HTIB) units you can commonly buy might look good at first due to being low priced however when you want to connect more inputs up to them (pc, consoles, dvd player after the original one dies) they can be quite a hassle. normally i advise people to get a speaker system which has a receiver unit instead of the combination receiver/dvd player combo box many htib's have unless they know the limitations of htib's and accept that.
My guess is that the +2 speakers over 7.1 would be either height or width speakers for the front. Either are just what they sound like, speakers that are place high above the main L/R or off to the side (11.1 would usually include both), however neither are discreet channels. Instead they are processed content from the front speakers to help give a larger soundfield. However, it could simply be a doubling of either the rear or side surrounds.
Without knowing exactly what you are looking at we can't give you a specific answer.