I've used both. They're very, very different amps. Both will get heavy. Both have a lot of gain. The Mark IV is "tighter" overall, I guess; it's not as bassy, at least. One isn't really any better suited for metal than another, they're just different flavors of high gain. The IV is very middy, and very clear sounding. The Recto is bottom heavy and is more of a "wall of sound"-type tone compared to the more focused Mark tone. The Rectifier is probably the more aggressive of the two. They sound huge in a band mix together.
In short:
Rectifier- Aggressive, bottom heavy, "huge" sounding
Mark IV- Clear, midrange heavy, "focused" sounding
Take your pick. Both do different things. And that's not to say a Recto can't sound tight and focused, or a Mark IV can't sound massive, 'cause they can. It's all about how you dial them in. But those are general characteristics.