Pro,
Before you run out and buy the expansion board for pianos, I would load the piano samples that came with the ES on cd. For my tastes the ones there are better than the onboard ones, like the s700 and jazz grands etc.

The one thing I sort of disagree with you on re: the Mo is pretty much an arranger with it's arps. The majore difference I see is that the Tyros with it's styles are like having several arps running at the same time if you will, one for a bass pattern, drums, lead voices, fills, rhythms etc. The Mo's arp with very few exceptions will only practically run one voice at a time. Even though in performance mode it could be applied to all 4 voices, it would sound like junk i.e. a piano arp playing the drums at the same time does not sound good at all.
Terry

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jam on,
Terry http://imjazzed.homestead.com/Index.html