Marshall is great for hard rock tone, Mesa is great for heavy metal.
That being said, I play in a '70s era-type hard rock band. I owned a 60 watt Marshall JCM 2000 TSL, and all the thing ever did was croak. The tone was OK, although the rhythm tone was really weak for what I wanted.
I returned it after it broke for the third time in a year and got a Mesa Boogie Single Rectifier. I had to take out a lot of unwanted bass, but now I've got a perfect tone for lead guitar playing. Kinda like '80s stuff but not cheasy or hollow sounding, but nice and full.
I can get a variety of tones with my Mesa, and it's also kind of neat because I can change the bias (with a switch) from SL6 to EL34 if I wanted a more "Marshall" tone.
As for playing live with huge amps, I've played live to audiences with tiny combos as well as half-stacks. The audience doesn't really seem to care or notice one way or another, just the other bands. Personally I'd rather laugh at them as they struggle with 8x10 and multiple 4x12 cabs to play a 20 minute set and let them think they get the last laugh when they see me bring my tiny rig. The small amps, in my opinion, always sound a helluva lot better because of the control you can get from them. My best sounding live rig was my Mesa Boogie through a 2x10 openback cab. My 4x12 sounds good for recording, but the 2x10 has a much better live sound.
-MD