It's a toss-up for me, honestly. Both amps are nearly polar-opposite in terms of high gain tones.
The 5150/6505 is the tightest and has more of a high mid sizzle than a Recto does. Compared to your Mark IV, it's more similar than the Recto would be.
I would never consider buying a 3-channel Rectifier again. They are a flabby, loose, muddy mess. Yuck. Do not want. Even boosted, they still don't have much note definition - forget using them for any kind of technical band or lead work. The last 3-channel Recto I had was probably the worst amp I've ever owned.
The 2-channel Recto, on the other hand, is much tighter and clearer. Could easily be used for leads and overall just has a huge, crushing tone. Considering you already have the Mark IV, I'd say a 2-channel would be the best compliment, as you'll likely find yourself dialing in similar tones on the 5150. Be advised - the cleans on 2-channels really aren't that much better than a 5150... but you already have the Mark IV for those. I prefer all Rectos with EL-34s or KT77s in the power section, helps to tighten them up even more, while giving them a little more bite in the midrange. They can get too dark sounding with 6L6s IMO.
FWIW, I've owned nearly every high gain amp on the market and the only amp I keep coming back to is the 5150. No way I'm selling my current one, you can pry it from my cold, dead hands