If weight isn't an issue, I'd go with the Randall RT2/50. If you want a more lightweight stereo tube solution, the Mesa 20/20 is a great way to go . . . I'm happy with mine, and while I've never tried the Marshall, all the research I did prior to deciding on the Mesa told me that the Mesa is hands down the better of the two . . .
Here's an off the wall suggestion - I got a 10 watt single-ended power amp custom made by MJW Amplification for around ?400-450.
The advantage over the 20/20 is that it uses one of the larger valves (5881 in my case) rather than EL84s. Felineguitars also has one (it was seeing/hearing his one that made me order one).
It sounds like a "big" amp, only you can use it in the "sweet spot" while at home without needing an attenuator or soundproofing.
I've used the 20/20 in the past and liked it. If it is a good offer i'd say to go for it. The el84's are great tubes as far as tone goes which is why it was at one time a favorite of mine. It is also a reliable amp which is in very stark contrast to everything else marshall makes.
I don't have any personal experience with the Marshall 20/20 myself...but when I was shopping/researching power amps, I read in numerous places that the Marshall 20/20 sounds good but is prone to overheating.
If you're still looking for a 1U 20/20, I've been running a mesa 20/20 for couple months now and absolutely love it; it does color the tones toward classic recto a bit, but I like that low-end sag feel from rectos anyway. I sold my RT2/50 purely to lighten the load but in the end I don't feel like I compromised on tone at all, and surprisingly don't miss the power reduction either; rack rig hangs right in there with my RM50 no problem.