George,
Excellent question. I Think Yamaha would really benefit from making a 76 key and 88 key version. That would really be awsome. Weight and size may come into play, but it should really affect it if the keys aren't weighted, but like that of the DGX series. I think your price of the two models is a good start.

Squeak
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.