Hi killerpiglet, I posted this on AA too, but since you're here as well....
I think the key thing is the type of music you listen to and what you want to get out of it. I have a Mesa Baron (although I'm not using it now). It can switch from full triode @55 watts to full pentode @150 watts, with stops at 1/3 triode and 2/3 triode. Over Christmas, my best friend, who is a professional guitar player, came over. He has formal training in classical and jazz as well as 'rock', and currenlty performs solo acoustic "new age" instrumentals. He's seen LOTS of the great guitar players live, in clubs and theaters, not stadiums. I have live CD's of many of these guitarists. As soon as I dialed in any triode at all, he said "That's NOT their live sound". Full pentode was it, even for his own CD, which was all acoustic guitar (I really thought he'd pick triode for that one). Full pentode in the Baron is a very aggressive and in your face sound with lots of 'slam'. For rock (using the term loosely) that's what you want. For an acoustic jazz trio, probably not.
I don't think that it's JUST a power difference. The Baron in 2/3 triode still puts out 120 watts I think, and that didn't cut it for the 'live sound'.