I'd say that it depends on how 'portable' you need to be.
I have a collection of MIDI synths/pianos, and they all work with an iPad for MIDI. (an old Casio Privia 88 key piano, new Roland Juno 76 note synth, 25 key Arturia mini keyboard, 49 key Alesis MIDI keyboard). I use Sibelius on my DAW, and have bought these various keyboards over the years for my work depending on where I'm working or my desk size. You can take the Arturia with you anywhere!
The little Arturia's are great and will fit in a laptop bag. You do have to be careful of your playing, because you really can't play anything beyond the current octave setting very easily unless you have a bar rest and you can hit the octave button while recording.
A 49 key MIDI controller is great. It can cover the range of a string quartet for all intents and general purpose. And, you can switch octaves when needed.
But, nothing beats being at your home studio with a 76 or 88 key setup for playing.