Another thought just to add in here, is you won't sound like another player whether you're trying or not. You're always going to sound like you (which is a great great thing), and your equipment shouldnt matter that much. Ultimately it's a tool. I play on a lot of different rigs, and i always sound like me no matter what. i have a pivture of Stevie ray Vaughan playing thru a solid state fender amp at a show, and i'd bet that that night there wasn't a person who noticed it wasn't his usual tube amp.
Basically, with enough experimentation and tweaking you can get the sound you desire with any gear that's even in the ballpark. those mesas and marshalls are nice. i'm sure you'll get the sound youre looking for with either. i buy way too much gear looking for the sound i need, and it's really wierd when i have that sound with my main strat and a no-name practice amp.
Also, which mesa are you looking at? I have a marshall, but im more familiar with the sound of mesas. the nomads are really nice, and the blue angels... wow...the single rectifier head has 50 watts and is a monster of an amp. the dual is more powerful and has more gain, and the triple has the ability to overdo it in every way imaginable. keep in mind amps sound best when theyre cranked up loud, so a 100 watt head cranked WILL be too loud for every purpose unless youre playing an arena or trying to deafen yourself. you may be better off getting a head that you'll be able to push harder. i'm much happier with my 45? watt fender bassman head than my 100 watt marshall (which by the way i'll sell for $300 if you want it). well, anyway, enough rambling. take care