Wirelessly posted (8130, Bell Mobility: BlackBerry8130/4.3.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/109)
It depends what you're looking for. The tethering speeds on the 8130 are faster. The camera is higher resolution and the OS has some nice changes built in. For the most part, however, you may find the devices to be very similar.
As John's indicated, coverage may be different since you'll be changing service providers.
Yup...spellcheck and an updated version from the 8100 of suretype (or whatever they call it). Video recording also.
I believe that all comes to the 8100 when the 4.5 OS is released, however.
So I think what you get different from the 8100 (aside from carrier/network differences) is going to be mostly HW related in the long run - higher res camera, easier access to Micro-SD card (on the side rather than under the battery), dual speaker slots rather than one, better (I think) orientation of camera/mirror/flash on back of phone (horizontal rather than vertical). Maybe a couple more things I'm forgetting...