I was going to get the 9 cell for mine, but stuck with the 6 cell for a number of reasons. Firstly, I use the laptop primarily at home and work, both of which I can plug it in and charge it quite easily. Second, the additional size/weight of a larger battery- I know it's not much more, but I'll be bringing it to school, along with my schoolbooks and whatnot.
Also, if need be, I can pick up a OEM 6 cell on
ebay for ~$50.
With the 6 cell though, I get almost 4 hours right now...it's actually pretty efficient. I've gotta find some battery optimizer programs and see if I can get more life out of it...