My old rig before the Pa588:
I sat in with a group last week and the keyboard player had one of those lower priced digital pianos. The action was OK, not as good as the upper end DP's but OK, the sound, well let's just say it was fine for the price. I had the idea to combine a low priced 88 note with an arranger module but I couldn't bring myself to buy one of them because I knew I wouldn't be satisfied with the sound.
While the Pa588 is on the heavier side of things, the plus of having everything built into a single piece of gear outweighs the extra weight. Not to mention the sound of the Pa588; everything from the
very good acoustic piano, to the electro-acoustic pianos, solo and ensemble brass and woodwinds, the orchestral sounds, basses and drums, they are all good as are the styles. And the flexibility of sound editing is really beyond belief for a so-called
arranger keyboard. As an example and since guitar sounds were mentioned in a post above, I was able to program a 2nd string unison bend effect by having one oscillator bend down by a half-step when pulling down on the lever. Starting with the lever down (toward you) then when you play a key, you hear two strings being played out of tune and by slowly releasing the lever, the lower pitched string bends into unison. At the same time moving the lever left or right applies pitch bend as it is normally programmed. This is an effect that can't even be pulled off on some of the true workstations available today. The only reason I bring this up is to point out the versatility and flexibility of the Pa588 and 500, 800 and Pa2X Pro.
Regarding velocity controlled sound aspects; Besides the master velocity curve in the Pa588, each individual oscillator, filter cut-off and resonance, amplitude and envelope generators that go along with them have adjustable velocity levels. You can customize each patch to your liking. The Pa588 is a fantastic instrument.
So all in all, price being pretty much the same, what is more important? Two lower end keyboards to carry along with a two tier stand, or one single keyboard that can do everything needed and sound better doing it?
Finally, a disclaimer. I can't stand it when someone thinks their instrument of choice is the only choice and everyone else are complete idiots for not agreeing. So as to not fall into that trap, my comments above are based on the comparison in the OP. Everyone has different tastes and needs, and like I always say; "It's great that there are so many choices out there so everyone can be happy".

Best Regards,
Dave
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Wm. David McMahan
LearnMyKeyboard
JazzItUp Band
The Modulators [This message has been edited by WDMcM (edited 08-30-2009).]