I have been gigging with the MR/Rectoverb25 for 2 years. I really love it. The volume has never been an issue because I use a Radial JDX into the PA, but the times I have gone without reinforcement at rehearsal or gigs, even with acoustic drums, it has been enough. It does push the clean channel to the limit, though. The Mark V seems to be as loud, though the EQ seems to sap volume from that channel. More on that later when I figure it out.
The Mark V Mini has a completely different tone. I think it covers a wider range, does classic tones well, does modern rock tones well. The EQ is a really helpful feature. I can add one to the Mini Rec, but it's really nice that it is built into the MV25. The cabclone is also a great feature.
I can't really say one is better, though the features in the MV25 win. I think that the MV25 is more versatile, but it does have it's characteristic tone. I can see the beauty of it, and I have a lot more playing to do with it before I figure it out.
Bottom line: I love the tone of both, and I don't think they overlap at all. Either could do any kind of gig or music, though each with its own character. The MR is more scratchy/punchy/rectifier-y, the Mark is Mark-y. Who could choose? I will likely continue to use both.
I was not joking when I wrote that I will use it as a stereo 4-channel Mark-tifier.
Last gig I did use the Mark V, with the MR as more of a backup amp. That's really because it is new. When I have run the Mark for a few rehearsals and gigs, I expect to go back and forth with the two and see what really works for me.
Given that the resale on the MR is pretty awful, I recommend picking one up cheap used as a complement/backup for the MV25.