I carefully compared both before settling on the Mesa, and it was the clear winner in the tone, flexibility and (reportedly) reliability departments... I thought the Marshall tone was a little thinner or at least less meaty and growly than the Mesa's, and I wasn't impressed with the Deep switch (tho' I love it on my JMP1); also, I hate the fact that you have to load both channels at all times (so you must ALWAYS use two
speaker cabs, or a huge stereo cab - no reduced loadout 1x12 gigs here...). The Marshalls also have some reported overheating problems that Mesa avoids with clever design ensuring superior air flow around the tubes... I wanted the Marshall 20/20 originally, because I thought it would mate nicely with my prefered preamp (a Marshall JMP1), but after my research and test drives, I chose the Mesa and have been very happy with that choice.