The major difference between the two bass modules is the size of the driver. Bass Module 500 = 5.25" vs Bass Module 700 = 10". I'd say it would really depend on the size of the room and I would recommend testing the experience yourself to make your own decision.
You can always pop into one of our stores and have a listen in the demo rooms.
If you have any other questions, please do not hesitate to reach back.
I own both the SB 500 and BM 500&700 i can definitely say there is a quite a big difference, the BM 700 has better lows (i mean room shaking lows) and just overall sounds better, the only thing that i found strange is that at lower volumes the BM 500 is a bit louder than the 700. if you can upgrade i'd say go for it, if you cant then youre not missing out on too much the BM 500 still sounds amazing especially for its size.