It doesn't seem like the br is that much more, so why not? Actually mb combos look nice to me, but the fact the br combos have a separate amp rackmounted in the enclosure is nice. You can take the amp out and use it with different cabs just by taking out 4 screws. The mb combos seem to have the amps in a "plate" form factor, so they probably can't be taken out. If that doesn't matter to you, I think the mb12 with the tweeter/mid/12" looks pretty cool.
I just bought a br510, and I really like it. It's really tight, clear and punchy. It's loud, and the eq options are really useful if you take the time to learn how to use them. To me, the compressor is surprisingly useable. The downside is that since the cab is sealed, the lowend isn't real awesome. I assume the br515 would be better in this respect. I've played the GKs and the Rumbles, and I liked the GK, but it wasn't the appropriate sound for what I do (it has personality), and the Rumbles had a "graininess" to the sound that I did not like. The Carvin doesn't have much personality, so it's up to your playing or pedals for that.