i think its a direct function of price-to-purpose ratio.
as you mentioned its for surround, depending upon how much are these speakers going to be used you can decide on how much you want to spend. obviously the 10 are more expensive than 9 , but does this extra money justify for the purpose you are buying them.
as far as the sound reproduction in concerned, i think the sound given out through the surround channel while watching a movie is about 5-10% of the total sound recorded. hence it hardly makes a difference.! for movie watching- the centre speaker is the most important, followed by the front L/R (these 3 should preferably be matched- i.e. same brand, same model line) then comes the surrounds.
just my opinion.