Wow. this goes on and on (although it echoes the mental gyrations that I can go through before, during and after buying a keyboard). This is a bit like the fable of the blind mean and the elephant - there are so many attributes one might consider. My first portable keyboard was an amplified reed organ with two voices which could be used individually or combined; only weighed 50 pounds, had a carry handle, and a clumsy direct valve linkage ... did I mention it was portable? Around the same time I was occasionally renting a Hammond L122 for gigs: usual classic tone-wheel sound, intermittent and sometimes noisy key contacts, limited soundscape in spite of the draw-bars, mechanical echo/sustain with microphonic tendencies, mediocre internal sound ... hard to cut through a rock band sound and a major PITA to lug around especially when stairs were involved; like many classic Hammonds, it was somewhat naked without an equally bulky Leslie cabinet but either way needed to be mic'd up (that approach didn't last long). Finally, joy (sort of): a Farfisa combo organ with flute and reed stops in a novel horizontal lever arrangement, sufficiently aggressive sound, somewhat versatile, now (judging by popular simulators) a popular classic/vintage sound, with a carry handle and only 70 pounds - at one point the instrument broke loose from the handle!; this needed separate amplification (more stuff to carry). Then came an equally 'portable' Yamaha YCD combo organ with usable foot-pedals, horizontal lever stops, a carry handle, needing external amplification, relatively nice sound - apparently another vintage/classic; this instrument was built tough: the detachable folding legs must weigh 40 lbs, swell pedal 5, foot pedals 20 ... so piece-wise portable and still works 40 years later. But Yamaha outdid themselves with the DX7: again a heavyweight with a steel package, endless flexibility if one has time and patience, some great sounds (I'm told that there are over 1/4 million patches somewhere out there but I'm willing to bet most load up 16 patches and rarely change them), somewhat awkwardly sprung keys that occasionally stick (but can be fixed once you figure out how); this instrument was built like the proverbial brick outhouse and just keeps on ticking - bought mine used 36 years ago and still a-okay. At the other end of the durability spectrum, a Novation synth which is light and has moderately versatile sound but I managed to break a key on day 1 (the shakeout was a collection of several tiny plastic bits i.e. not repairable).
The parameters are many:
- sound quality
- sound diversity/versatility
- sound verite'
- durability
- keyboard feel
- keyboard action
- portability
- ease of use
- amplification and speakers, if any
- price
- etc
Choice is a balancing act between the many factors. As we see in this forum, different people weigh different parameters more highly. In quality management a rule is 'avoid local optimization' but maybe Sir Mick said it best: 'you can't always get what you want'. I once tried a Kurzweil digital baby grand that touched all the bases, IMHO, but ... improbable to justify the cost and floor-space if I brought one home (my Wurlitzer console organ with full pedal-board seems to have used up my credit in that area).