Palo, my friend HankB has an E50, and he loves it. I have a S900 and I like it a lot also.
The E50, has better drum sounds--that LIVE sound that Yamaha hasn't figured out yet.
However, the E50 does not have vocal processing.
The S900 has a lot more features, such as a recorder that will record your performance, including vocals. It has a great vocal processor and vocal harmonizer, with elaborate editing possibilities.
To me, they are both good keyboards. They just offer different things. Of course there is also a little difference in price.
The key feel on the S900 is very close to the others PSRs - very light. The E50 has a sturdier feel to it.
Yamaha uses pitch bend and modulations wheels--Roland uses Joystick. I can work with either, but I would prefer the joystick.
Hope this helps. If you have any specific questions, I'm sure Hank will read this also, and chime in.
DonM
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DonM