Hi,
I agree with FRUCTAL and Nocki,
those amps are both very good amps and they both are very versatile.
But the quality of tone is way better in the Quad. Breath, life and dynamics are in it´s own class.
A MK IV Combo is almost completed (missing only effects), small but heavy. A Rack with a Quad needs a power amp (+effects if needed) and separate cabs. That makes it bigger and more heavy in total, but you can splitt the units and the weight. This can be an importand point to think over.
The quiality of the poweramp in a Mark IV is verry high and the switching possibilities offer a lot of colours to the sound. So it depends on the Poweramp you choose, to compare the two possibilities.
295 is very good 2:90 also is and, not to be forgotten: VHT 2:50:2
Maybe a Mesa 20:20 is an issue, too.
Do you realy need stereo?
Take a look at the posts about FX:
http://forum.grailtone.com/viewtopic.php?t=3434&sid=247b223ae67ee02af4f976d068ecda40
If you look at the preamp only ... no question: Quad is in the lead.