I've only looked at the documentation for the BT tool but it appears all it will do is register a like for like battery. Same size and type. With NCSExpert, you could change battery type and size. It also costs less. I would also bet the BT tool is easier to use but NCSExpert is not terrible. You just have to follow directions exactly even though they aren't very user friendly. Essentially you plug into the OBDII, set the car to on, download the module you want to change, make the changes, and then upload the module. The menu selections you make are in German, however, so I need detailed instructions.
128i Convertible, MT, Alpine White, Black Top, Taupe Leatherette, Walnut, Sport
Ordered 5/22/09, Completed 6/4/09, At Port 6/9/09, On the Georgia Highway 6/13/09, Ship Arrived Charleston 6/24/09 at 10pm, PCD 7/21/09