Completely different championships.
GO FOR BOTH Mk.4 + Prophesy!
I have a Carvin Quad-X Amp (which is sort of a Mk.IV) and a Rocktron Chameleon (the Prophesy's predecessor).
They have different features and purposes, so... I won't sell either!
The Quad-X is paired with a Peavey Classic 50/50 EL84 power amp and is my main rig (some FX processors in the loops also).
The Chameleon (since it is FX embedded) is paired with a Rocktron Velocity 100 solid-state power amp and is my backup/practice/home studio rig.
I prefer having a full digital/solid-state backup rig just in case something goes wrong with the all-tube rig and/or there is some sort of problem with the mains power of the gig.
That being, and in your situation, you bet I'd go with both!!!
I know the Mk.4 really well. It's a really versatile amp, lots of gain, excellent tone. Never got my hands on the Mk.3... but I think the Mk.4 sports the same gain structure as the Mk.3.
Never tried the Prophesy as well, but I know well both Voodu Valve and Chameleon, its predecessors, and I also know the features of the Prophesy. It's Rocktron, what else can I say?
Huge (but I mean *HUGE*) gain, fantastic effects, 200% versatility. You'll never get to use it 100% of its capabilities.