b90, I'm gonna piggyback on Yahboy's comments here, and they're good ones. The two Ampeg combos will be similar in the amount of volume they put out, but the 115's bottom is gonna be way bigger, and yes, maybe boomier as well. That's why I, like many others, went with the 112. I'm mainly using it in my studio and at school, but if I was mostly playing out I could see using the 115 and letting that booty fly!
Compared to my late Rumble 100, as Yahboy said, it lacks the Fender's natural boxiness and lack of mids where you can actually hear those frequencies on your bass. It literally took me 15 minutes, when comparing the two, to realize my ears were much happier with all my basses.
Finally, the SGT, though everyone seems to wish it to be a gain knob, isn't. It adds (in moderation) some once-sounding grit to the amp's clean tone and gives it a much more musical sound when compared to the Rumble's overdrive feature, and ten times better than Ampeg's Scrambler circuit!
So knocking the 115 to those bass brothas of ours who chose it, but as was with the BA v1 line, the 112 seems to be more musical and easier to score better sounds with than any of the others in the Rocket Bass series. That, and its popular price point should make it a runaway bestseller. Enjoy yours!