I currently run a head switching rig, which includes a Mesa Racktifier and a Mesa Mark IV. It's really the best of both worlds...the rectifier and mark series amplifiers are quite different.
Long story short, the Rectifier makes the perfect rhythm amp, and the Mark IV does amazing leads (due to its more fluid response) and cleans.
If I had to have one amp to due it all, it would be the Mark IV, as it is a lot more versatile...the lead channel can handle both rhythm and leads. BUT, the Racktifier has an extended presence and kick in the low end that is just meaner for the punishing rhythm work.
I used to own a Roadster...simply put, it is a darker and smoother rectifier. My rackto, and older 2 channel rectifiers in general, iare MUCH tighter and punchier. The smoothness of the Roadster does, however, give to its versatility. It has an AWESOME clean, and can pull off good leads too, which are usually inexistent in the stiff response of the rectifier series. The typical rectifier high gain sound is a little more refined though, and not quite as aggressive.
In my case, it was worth getting the two 'simpler' amps that cover my rhythm, clean, and lead sounds PERFECTLY between them, rather than compromise for the one amp that does more, but not quite as well. The cost of the two used is about what a new Roadster would cost too
P.S. No experience with the Mark V, so can't help there.