i recently had to purchase a new head and i went through tons of info about new gear. from my experience, and personal taste, i found mesa to be a better buy.
my friend opened up his studio and he took me along to advise him. this was before i knew what i was doing. we bought a marshall dual super lead (DSL?) $1500 and a 4x12 cab. $900
a couple of months later i was loaned a fender bassman head $400 and a 2x12. $699 running a tube screamer $200 through it impressed my friends way more than the marshall TSL.
soon after i bought a mesa boogie F-100 head $1700. this surpassed both of the previous amps by a longshot.
and recently again i bought a mesa single rectifier $1900 and i've found my sound.
i would strongly suggest a mesa for any pro player simply because of their diversity of sound, however, choosing an amp depends on the style you play.
to my own personal ears:
the marshall DSL had a great AC/DC sound, but only on the first distortion setting, the extra distortions on it took away flavour from the sound. i couldn't get a guns n roses sound out if it either, i think it has to do with slash's guitar being equipped with whatever pickups he has. so if you are set on marshall, save yourself the money and focus on an amp with only one distortion setting, like an JCM800?
the fender bassman teamed with the tube screamer and any other distortion pedal (NOT the metalzone, lol) shoud sound great and give you sounds like the rolling stones, SRV, Clapton and any other classic rock band. these amps are inexpensive and easy to find. a good 2x12 is harder to find. i was using an oversized mesa 2x12.
the two mesa's were able to give me the grit and the smoothness i needed for my original band. very brit rock sorta feel.
i could sound like the edge, or santana, or jimmy paige.
specifically, the F-100 gives me a great bluesy zeppelin feel, but it didn't have a third volume boost for leads, you would have to rely on your volume knob on the guitar.
the rectifier gives me a very modern sound, there's alot of brightness that comes off as almost FUZZ as compared to other amps and i think that would be true of all the rectifiers.
i noticed that the F-100 had a raw feel for leads as compared to the rectifier that had the layering distortion that fills the rhythm tracks.
anyways, i'll let you guys know more as i learn more. if you want to know.