Ok, I think I'm getting a clearer picture now.
In the past I have used a Korg Triton Le61 for the type of purpose you seem to require.
It's light weight & wide array of sounds and features make it ideal for just tucking under your
arm & away you go!
Something that has been an issue on occasions, though, is the
size of the keyboard.
Octave shifting while amusing yourself at home isn't too much of a problem, but in a live
(or indeed rehearsal) situation can prove to be one of those niggles that leads to frustration,
not only for yourself, but to others aswell - when you suddenly realise you've run out of keys
and have to pause/re-start to get the 'board in the correct 'range'.
You began by asking about an 88-note keyboard, so the jump down to 61-note (ie: MM6) is not something I'd recommend.
I would suggest at least 76-note to ensure that little more 'elbow room'.
Yamaha's MM-series is only available in 61 or 88-note versions (88-note weighs
a lofty 34.4lbs / 15.6 kilos) and does not have aftertouch.
Have a look at Korg TR-76 (the successor to the Le)
www.korg.com/TR... it's keyboard is
velocity sensitive
and has aftertouch and it comes in at only 20lbs / 9 kilos.
It's a cracking keyboard for the money.
I don't have anything bad to say about Korg & as the TR has reached the end of it's
production run you can pick them up for a good price, especially on the second-hand market.
Good luck & let me know how you go on, yeh?