Have the opportunity to use an M1 or U20 for live gigs, which do you recommend and por - que [why]?

It depends a lot on what sort of sounds you will be needing for the performance. If standard sample based sounds such as piano are what you will be using, then I'd probably recommend the U20 over the M1. While the M1 is great for atmospheric pads, organs, brass etc. the samples are actually quite lo-fi and rely heavily on the FX onboard the M1. The M1 piano, while being well known, was not a particular favorite. The U20 is a lot more straight forward in terms of the types of sounds offered in the presets so is probably a better if it is to be the only keyboard you will be using.

I'm not so sure that this is helpful in your decision, but I have a Roland U20 and I am NOT disappointed in it's sound quality live on stage. I just recently did a solo fly on New Year's Eve live on stage with the sound panning randomly left and right and it was absolutely the most haunting thing I've ever heard! Several audience people were stunned into silence. I really like this board a lot, even with just the standard presets being used as I don't have any cards around to plug in and create new patches with.

I remember that random pan thing. It was great with a Rhodes sound too! Ahh, memories!

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Uncle Dave