I have listened to ST-10, ST-10M and the new Ref 21 (name to be changed) before Christmas holiday at the factory.
Until ST-10M and Ref 21, ST-10 is 2nd best amp after Ref 20.
ST-10M has better control than ST-10 not simply due to the extra power, but the amp module has some changes, plus different capacitor bank. Keep in mind that going with this configuration will cost double. We have a standing offer to convert ST-10 to ST-10M for $250 USD, so if you are not sure if you really need ST-10M, you can defer it until budget allows.
Ref 21, due to its 1M Ohm input impedance (ST-10M does not have 1M Ohm input, I posted incorrectly in another thread) seems effortless. It is hard to explain until you hear it. bigger sound stage, more details, even better control, etc. If you already have Ref 20, there is no point trading in for Ref 21.