Yes, the extra time IS for the extra RAM.
Keep your eyes on Yamaha's NEXT generation of high-end arrangers to hopefully implement the USB2, Firewire AND ethernet connectivity just starting to appear this year on their new MotifXS line.
Until this is implemented, sampling in arranger use is of limited use except for small, speciality sounds because of the 'any song, any style, any time' nature of arranger playing. If you are not playing live, OK, it's bearable (just!) but unless your only need is a few specific sounds for the whole evening, you are better off without it...
(And don't forget that UPS power supply - you can't afford 10+ minutes of down-time after a power drop or brownout.....)
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An arranger is just a tool. What matters is what you build with it..!