I would like both, but would buy WiFi over GPS.
I have a Garmin nuvi, and it can work in places that mobile coverage may not go. GPS-on-board-Blackberry would be convenient for travel in areas with dense cellular service coverage. That would allow me to travel without the nuvi on most of my business travel.
The WiFi with TMO UMA is a higher priority because of poor cell coverage in some locations that I work, and because of international travel where I can usually bring along a router.
I'd like to have a device with both WiFi and GPS, but the availability of a WiFi/UMA model on TMO will be enough for me to change from my 8700.