The Dual is looser than the Rectoverb in any mode other than Bold/Diode, not tighter. (Which if you want a bigger contrast to the Mark, is what you need.)
I disagree that the difference is minimal. The Rectoverb is a very limited version of the Rectifier sound - you get the Recto preamp voicing, but it doesn't have the four-tube complexity, the tube-rectifier dynamics, the Spongy mode, the effortless massive tone, and the reverb is pretty crap (and solid state). It's very much the poor relation in my opinion, it sounds thin and a bit sterile by comparison. The Dual is easily worth the extra €250 - if you really want to make the Dual sound like the single you can (pull two tubes, set to Bold and solid-state rectifier), but if you want to make the Rectoverb sound like the Dual, you can't. You're missing most of the features that make the Dual so great.
OK, it's not strictly a fair comparison since the Tremoverb is a 2-channel rectifier with reverb (proper tube reverb, one of the nicest on any amp) and tremolo, not the same as a 3-channel, but the Tremoverb is my favourite amp ever, and I don't even like the Rectoverb enough that I would own one if it was the only choice. They're that different.