I have SB2 and was also thinking about moving to the SB3. I had and was use to my old Paradigm mini monitors with great bass that wasn't there with the SB2.
I heard the SB3 which have more bass, but I wouldn't say it is worth the extra $200. Decided to purchase a HSU sub and extremly happy with my decision.
I would put the extra $200 toward a better sub. But, you really can't go wrong either way.