I was wondering why some guitarists often use a separate Pre and power amp as opposed to just a head. Is it more versatile?

I've been planning to get a new amp for some time... Really looking for that Petrucci type rhythm tone (and maybe lead) and there's some things I've been concidering. A Mesa Boogie Triaxis preamp with a 2:90 power amp.. or maybe a mark IV or a IIC+. (if I can even grab any of this off Ebay). My band really wants me to get a 5150 for some reason but I don't think that's really the sound I'm looking for.

Is the "preamp/power amp" a better, more versatile choice over just a head? Can the triaxis mimic the Mark sound pretty well?
Wouldn't have a clue, although the only reason i can think of that would be better is that it'll cost you a lot less then buying a head for the same sound.
The Triaxis is a great preamp and extremely versatile. Couple that with the 2:90 and your gonna have yourself one hell of a rig...

And heres a snippet of the Triaxis's modes...

"Eight separate modes deliver the elusive creamy gain of the Mark I, the heralded focus of the Mark IIC+, the scooped Rhythm of the Mark IV™, and the heinous crunch of the Rectifier™. But these are just a few of the classic sounds at your fingertips."