Having owned both players, I'll give you my thoughts:
-stability - the Gigabeat edges this one. I've had a lot more crashes on the iPod.
-battery life - agreed, since the recent changes as regards disabling iPod hardware.
-capacity - the iPod is clearly superior here, depends on how much you'll want to use as purely music or if you want to have half the capacity free to use as a mobile HDD.
-interface - I find the iPod to be very poor from a usability POV. I'd never owned one before about a month ago and have never used the OF, but certainly within Rockbox it's taking me a LONG time to get used to how quickly the scrollwheel scrolls. I always end up overshooting the selection I'm trying to make, it's pretty frustrating.
Having said that, if you're used to the scrollwheel then YMMV.
-size and shape - the iPod is undeniably sleek
-compatibility - the GB also have USB-B on the unit itself, you don't have to plug it into the dock. There might be support for the apple accessory protocol in the near future, which would mean that more of the myriad accessories for iPods would work as expected.
-price - the GB is going to win this battle!
Good luck, and the final point is that IMHO either player without Rockbox is virtually worthless.