I've owned a good number of both Mesa and Ampeg cabs (prog/tech metal player in bands since like '99). Ampeg if you are on budget (used fridge for $500 or less if you are lucky) or like traditional/tried & true in terms of tone preference. Mesa cabs are brighter, clearer, and more direct sounding IMO. The current PH line has an awesome crossover technology that allows another level of tone control. I find Mesa cabs to be a big improvement for Ampeg heads to get a more aggressive tone... takes the blanket of off 'em.
I owned a SWR 8x10 Megoliath for a while... it had a cool metal grill. Ultimate blues/reggae 8x10 if you ask me. I did not have the super burly low mids that I like but did whale-sized sub lows and general punch in the most epic manner.
I wouldnt risk it on Emperor bass cabs. I think they just pulled some dimensions/numbers out of the air when designing those cabs, many people have found them to be questionable. Bass cabs are science now. I f-ing love their guitar cabs (own one, just ordered another) and have had a few other top of the line guitar cabs for a point of reference.