I read somewhere (a long time ago) that BBMaps uses vector graphics so it uses less data than Google Maps, which uses .bmp images. I'm not sure that's true though.
I have never tracked data usage of them either. I have an unlimited data plan.
I prefer G Maps for its street view option. BB Maps is good and getting better all the time. If G Maps had pinch zoom on the Torch it would probably be no contest for me.
I have both. G Maps also shows icons for businesses on the map too. Rather than "search" I can browse the map and see restaurants, shopping, etc, tap the icon and pull up the listing.
BB Maps does seem to draw faster when I'm driving and out of 3G range.
Don't forget that BlackBerry Maps will use far less data than Google Maps because of data compression. Also BlackBerry Maps usage comes within the BB Data plan, which as I understand it Google Maps does not.
The quickest loading Maps app I have found is Ultimate Maps from App World. Shows a lot of detail of businesses and is more accurate than Google Maps which has a habit of labelling the wrong building.