I would always recommend quality over size. But I would also recommend making sure the two strands you're comparing are coming from the same source, otherwise the quality question won't matter.
9-10mm would be a wonderful size but I think I would prefer 8-8.5 AAA, or even better, I would try and get 8.5-9.0mm AAA. I find that a very good size, it looks substantially bigger than 8.0-8.5mm.
I would definitely go with slightly smaller AAA pearls over size any day. The cleaner surfaces, improved luster and (if FW) the rounder shape all combine to be more pleasing to the eye in my opinion.
The AAA, because 8 to 8.5 is a big enough size to enjoy. I agree that if you could go up a size, but keep the quality - even better. Make sure there is a good return policy - just in case.