From a technical standpoint:
Rectifier has "better" distortion. The Mark series doesn't have a cold cathode distortion stage, so the distortion is a bit more raw and grainy. Now, you might call a Rectifier grainy or fizzy, but it's for other reasons. Namely, the Rectifier doesn't have a negative feedback loop on Modern Red, which contributes to the amp's feel of being heavy and brutal (and fizzy with the wrong setup!). Of course, many complain that the Rectifier isn't tight. It's because you need to turn your amp up! Already tried that? Well, there's a capacitor that cuts out some high end on the Rectifier that isn't there on the SLO or 5150. Halve the value in the Rec and you'll be amazed at how much tighter the Rectifier gets.
I'm not saying the Mark series might not be "better". But I am saying that without a cold biased preamp distortion stage, the content of the distortion is far more filled with odd order harmonics than even order, the opposite of what our ears find "pleasing".
From an experience standpoint:
I love Rectifiers and while I've never bonded with a Mark I've played, I've heard some excellent recordings. Since you're curious about Rectifiers, I'll talk about them. I think too many people don't understand that the amp is NOT your rig. Everything - from your pickups, to your guitars, cabs, and speakers - is your rig. The amp is only a part. Because of the way the Rectifier is designed, it's very important to match it with an appropriate cab. Maybe it's a design flaw and the Rec should work better with other cabs, but then it wouldn't sound the way it does.
But even that isn't enough. Rectifiers - not very unlike the best Marshalls - must be turned up a bit to feel the way they're supposed to. Once the speakers start working, the feel of the amp improves immensely. And I would in fact argue that because the Rectifier has so little gain compared to some modern amps - Bogner Uberschall, Diezel Herbert, or the new Egnater Armageddon - it in fact is not good at covering up mistakes.