The 9 has a higher R value, so should in theory be warmer. It's also heavier and bulkier.
I use a 7 and it seems plenty warm to me, although I might pick up one of the 9s just to try it out. At those prices it's hard to beat. I believe my local REI has some of the 9 standard in stock and on clearance.
I usually inflate my downmat about 3/4 of the way and find that very comfortable. My fiance, however, prefers hers fully inflated in the hammock, so it's a matter of personal preference for comfort rather than what you're "supposed" to do. In general, the less air you put in it, the better it will conform to the shape of the hammock, while the more air you put in, the warmer it will be. The trick is finding a balance that suits you.
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