The SPSB-10 and SPSB-11 both use 12v batteries (probably of the 23A variety) whereas the SPSB-8 uses normal 9v batteries. Not only are 9v generally easier to find, but one will be able to power your mics for a few hundred hours easily. The 12v is rated for around 30-50 hours of mic power. You'll be buying a lot more 12v batteries.
I would've thought at first that the salesman was pushing you towards the 12v BB due to smaller size, but looking at the specs of both, they are almost the same size with the 9v being just a little wider. Unless someone else has information, I don't know why the salesman would push you toward the 12v one. Ultimately the choice is up to you.