The T100 & the B100 are the same vehicle other than the body, body mounts, and wheels. Since you already own the T100, then go ahead for the B100 this time around. You can always take the parts off of your T100 and convert it later if you like having the truck better. But as the buggy, the pin tires that it comes with have more "bite" than the chevron tires that the truck comes with. Also, when you get the buggy, you get that cool wing!
I own 3 T100's, but that's because Radio Shack was sold out of the B100's. I'm actually thinking about going back to pick up the buggy one of these days!