Hey I thought I'd chime in on this since I just purchased 40 9v rechargeables and two 10-unit chargers for use at work in a pro-audio situation...
I talked to a really knowledgeable person at thomas distributing about what I wanted them for and that I wanted the longest possible run time.
He recommended the Tysonic 250 mAh, 9.2 volt battery on this page:
http://www.thomas-distributing.com/9v-rechargeable-batteries.phpThe powerex ones are either 8.4 at 300 mAh or 9.6v at 230 mAh.
Some devices don't play nice with 9.6v batteries, and the guy told me that they are a bit longer than your standard 9v which could be a problem for a lot of pro audio uses.
I ended up buying 20 powerex 300 mAh and 20 of the Tysonics.
They run our devices (wireless clear-com communication boxes) for close to the same amount of time...with the Tysonics lasting a little longer. They still run about 2 hours less than the energizer industrials we used to use, but that's ok in our situation.
I'd recommend the Tysonics. They are actually 9.2volts, and are more mAh than most others. Seems like a good balance or voltage and longevity. (and they are about 1/2 the price of the lithiums!)
YMMV and good luck!
/jamos