I have the 8800 and 8300 (Curve) on my desk right now.

The 8800 has the built-in GPS, and if you download the latest version of Telnav (v5.1) - it actually isn't bad, better than the one that shipped with the 8800. Unlike some of the others, I have found the 8800 to be pretty sensitive, I'm guessing they use a SiRF chipset in it. A big plus would have been having the voice directions coming over the bluetooth headset, I've had it work by accident a few times, I start a call when the thing is navigating and then it continues to use the headset even after the call is done. In the car over the unit's speaker it is hard to understand what the heck it is saying sometimes.

The 8300 is a smaller form factor, which I like, and easier keyboard for bigger hands. Doesn't have the built-in GPS, which is a bummer, but you can use an external puck, the $99 one they sell at the Cingular store is really good, but then you are sucking more battery out of your unit using bluetooth.

Of course they can't hold a candle to something like a Garmin Nuvi, but the times I've used the 8800 was when I'm out in the car and realize I need to find a place and grab it out of my pocket. Luckily this was before the 8300, as I find I forget the GPS puck frequently if I don't have my work bag with me.

Functionally, they are almost identical, the spell check on the 8300 is nice, keyboard is better for me. I tested the XM radio for the 3-days on the Curve and had lots of problems, even in major cities, keeping the stream stay continuous, it is a great idea, but until 3G comes to the Blackberry, probably not worth the money.

Both are great units, can't go wrong either way. Try the keyboard of the 8800 first, if you can deal with it, probably worth it for the GPS.