I am using 4x10s with my 50w plexi. Totally happy with it. Might be a little more punchy than I'd prefer, but its a really nice size and weight. I'd actually pack up two, and not be too put out, as opposed to lugging around a 4x12. (I have two Marshall 1965b cabs.)
BigMike put the thought in my head that I'd get a more open and less compressed tone with a 4x12. I believe him. I'll probably get one some day, but for now, I'm okay with the 1965s.
I recently picked up a 4x10 Avatar / G10 Vintage cab on Craigslist for a song. It's lightweight, really easy to carry; almost as light as the head (somewhere around 35-40lbs). Bought it to pair with a Music Man 130W head. The combination screams. Punchy, crunchy, sounds really fantastic. Hard rock up the wazoo. I recommend highly.
I play a 1965a with my JTM45.. Love the tone. I gave up lugging 4x12's 10 years ago after rupturing a disc while lifting one after a long night of playing. It's not the weight - it's the size that makes it difficult.
I can carry the 4x10 with one hand. It fits in a car seat easily.
Unless you have a crew, 4x10 or 2x12 is really the only way to go, IMHO.
Modded Ceriatone plexi 50 with Marshall 1965a cabinet. Loaded with Celestion G10 Greenbacks.
Changed the grill cloth to large check. Not a perfect job, though. I will straighten it out later.
Punches like Tyson circa '88. Easy to carry around.
I've run a Marshall 4x10" cab with Bassmans, a Bandmaster, Traynor YSR-1 and an Ampeg B25. Great sounding cab. A lot punchier and more immediate than a 4x12". It's also a good bit lighter and smaller. Much easier to haul around.