Actually the Radium does not have aftertouch. It has velocity. Aftertouch is applied by a slider on the Radium. Not very musical.
Velocity sensing is how hard you play the note from one strike to the next. It is a form of modulation (usually called brightness)
Aftertouch is the pressure that is applied after the note is struck and the key is held down.
Polyphonic aftertouch is where each key has it's own sensor so you could modulate one note out of a chord. AFAIK only Ensoniq(R.I.P.) had that feature in their keyboards.
Most new controllers out there don't have aftertouch.
The Novation controllers DO have aftertouch.
The new Alesis Photons DO have aftertouch.
The new EMU controllers DO have aftertouch.
The new KORGs DO NOT have aftertouch. (Though the pads do, go figure).
All Evolution and MAudio controllers DO NOT have aftertouch. It's a shame the the Keystation 88 does not, what a waste of weighted keys.
As a classically trained player who is deep into synthesis, aftertouch is a must for expressive playing.
One negative side effect of beat looped music is that the products that cater to them (25, 37, 49 key controllers) mostly do not have aftertouch.
I need a smaller keyboard to play synth parts, and I do not feel like carrying 2 full sized boards to a gig. That's why I'n excited to see how the EMU and Alesis feel.
Most softsynths worth their weight in salt can be programmed to respond to aftertouch. It should be available as a modulation parameter.
HTH,
Prana