I've also read that S30 has just one PLG slot but i'm not sure. But external power supply ????
I hate them in rack modules but on a synth ?
Anyway S80 is excellent and has Yamaha's signature to its voices. As you told before Roland's sound is warmer but S80 has it's own personality...
As for having four slots. Then S80 would be the best multi-synth-engine multi-everything
great sounding, master synth around.
By the way S80 using PLG150-PF has the best piano & EP sound among current keyboards (with the exception of kurzweil and XV