I can't offer a full comparison but I can give half of the puzzle.
I have a 17" 2.8 GHz MBP I just got a few days ago; so keep in mind the battery is brand new with just a couple of charge cycles.
I work from home on this computer as a system administrator. As such, I utilize LaunchBar, Mail, Safari, Adium, TweetDeck, VoiceMac, iCal, iTunes, and Terminal constantly on WiFi.
I calibrated my battery yesterday and with brightness auto-adjusting and the backlight on 50%, the laptop lasted approximately 8 hours.
On a separate occasion the day I got it, the battery lasted about 2-2 1/2 hours with 100% brightness and 100% backlight while playing WoW and listening to iTunes.
All in all, I am very pleased with the battery life. I would definitely go with the 2.8 GHz if you can afford it; after all, the processor is one of the things you can't upgrade as easily (if at all).