Have opportunity to use eithe in live applications. Which would you recommend and why?

I've actually answered this for you on the Roland Forum where you also posted it. Briefly, my own opinion was that for a live performance where you'd just have the one, I'd select the U20 simply due to the fact that for standard sounds such as piano etc. the samples are cleaner than the M1.

If I was using it at home or in the studio and I had the standard pianos etc. covered already with other gear, then I'd pick the M1 in an instant because despites it's warts, the sound is warmer and it's really a lot more fun to play than the U20, mainly due to the dual FX processor onboard which is an integral part of the M1's character.

I'd add that the M1's sequencer, while rather archaic by later standards, still has some creative punch for the dedicated player.

The U20 didn't fly very far, but its module version, the U220, seems to have a solid, low-key popularity with a lot of pros. The U20 seemed like a D-50, but much more crisp in certain ways. Its FX are more limited than the M1's, but they sound fine and there's a slot around back for sound cards, which can still be found with minimal difficulty. And did that sucker have a rudimentary arppegiator on it or am I just having a flashback?

Be aware of the delicate nature of RAM/ROM cards if you go that route. You can always buy a U220 for your M1...I buy tons of gear all the time with the money I make as a lawyer, cowboy, firefighter and astronaut. Seriously, its a proven workhorse module; don't discard it too casually if the price is right.