They are pretty similar, in truth. Physically different battery sizes, though the IPro and BPro batteries notcigs sells test out as about the same real mAh, so same run time. Same firing button, same electronics, same wheel voltage adjustment, both have endcaps with a delrin thread piece, which is used as a positive Off. Otherwise a slight difference in the drip well size, but that's about it. IPro is longer, slightly skinnier. BPro wider base lets it sit on its endcap maybe a shade better, and I like the BPro form factor with a tank.
The IPro batteries are protected, I believe the BPro Powerizers have some protection, but perhaps not the full classic set that the IPro batteries do. With IPro you can use other protected ICR batteries, with BPro you can use protected ICR, and AW also sells an IMR 550 mAh in that size.
They've both been in production long enough to have any early kinks worked out.