| There's no venue I've ever played or plan to play where a single fifteen in a tiny box would've or will ever suffice. That pretty much drops the trap door out from under the GK, 'cause it can't drive an external speaker. But if I were playing only tiny rooms at low volume, or if I were looking for something to serve as a personal stage monitor because the mains were handling the BG, then I'd probably be okay with either one. If you want a crisper, flatter sound, minus the typical GK character, you're better off with the Carvin -- just right for the bippity-bappity-slappity players. With its tone controls set flat it has roughly the tone of a 1962 portable transistor radio. (I am not exaggerating.) But you can get some bass out of it if you cut back a few dB around 120-150 Hz to fix the small-box response peak (at least that's what I modeled and what I hear from mine) and crank the hell out of the bass control and use the high parametric to kneecap the midrange (and maybe use the contour if required). Sorry I can't help you with the characteristic tone of the GK combo, but GK heads are bassy when set "flat," so it would stand to reason you could coax more bass out of the GK than you could out of the Carvin. Finally, check the feature sets and see which unit has the one or two you must have.
I realize this pile I've left here is inconclusive, and I'm sure I've missed a couple of points, but maybe one or two things will help you. Also there's a long MB15/12/10 thread on the board if you feel like chasing it down. I did a review, over several replies in that thread, of the MB15, and I WinISD-modeled the speaker inside the enclosure. | 
