The level of realism and selection of the sounds-
each keyboard typically does some sounds better than others so decide
which main sounds are important to you and play each model (or listen to sound files)
you are interested in to find the one that sounds best to you. Does it have all the
sounds you are looking for to make a completed song?
Ease of use- hardware knobs, buttons,... amount and location and logically placed
Display size- touchscreen, color, icon based, easy to read, backlighting not too dark,...
Sequencer- depth of editing features, ease of use as you could spend most of your time here, linear recording only or pattern based recording as well, ...
Quality and number of effects- reverb, EQ per part, compressors, master FX, number of part insert FX, master/global FX, does it have enough DSP to finish a 16 track song without needing external processing
Audio recording/sampling -2 to 8 tracks or more- how easy is it to actually record and playback, does it have enough recording memory/time- re: roughly 10 meg a stereo minute, how is the recording backed up,..
Drums- does it have ‘easy to chain’ patterns or is the arpegiator easy to use, does it have pads on top of keyboard or none at all,...
The number of simultaneous notes the keyboard will play (polyphony)- important if you write denser arrangements or use instruments that use a lot of polyphony (piano,...)
Control computer software via hardware sliders, knobs,...
Computer control the keyboard with editor/librarian- as VST plug-in,
standalone, 100% editing, the visuals on a big
computer screen can shorten the learning curve (much more intuitive)
Operating system- is it intuitive or obtuse, ...
Maximum RAM memory- will determine possibly how much recording time is available
or how much room you will have for adding external samples/sounds
Connectors- # of audio outputs, balanced or unbalanced input,
data storage- memory card, jump drive, USB to computer connection