
The Mesa is going to have a much heavier tight low end. I've owned both as well as everything else under the sun and my favorites are the Mesa and Fenders (which I currently play).
good choice... GREAT cabs.
Do not overlook Eden Bass gear, their cabinets are amazing. Seriously massive output and really tight low-mids / low lows. From a tone perspective, in my opinion you'd be better off with a 4x10 + 1x15 or something. I've never heard an Ampeg 8x10 sound good live, just to much massive low end.
Mesa
As GreySkull has said, you'll hate carrying an 8x10 and you said the 6x10 has the flight case...