I have played both the P250 and the S90 they are both very fine high quality keyboards. That P120dUde is smoking crack if he thinks the P250 is "awful" compared to teh P120. It one thing to have a personal preference of the keyboard you like but its another thing to mindlessly slam and not respect another keyboards and people's opinions.
If you dont need the built in speakers I would go for the S90. The action should be almost identical and it will be smaller and more expandable for the future. The P250 has a very beautiful grand piano sound. I like it on the mellow grand piano because its so warm and nice. But it has several pianos. I would LOVE to have either the P250 or S90 or even P120, P90, P60.
Many people think the P250 has the best digital piano sound of the yamahas. I would not disagree with that. Its a really good sound. I would definitely recommend trying both the P250 and the S90 in the same showroom and decide which would fit your needs the best.
People like Yamahas, Rolands, Kurzweils, etc. Get the keyboard you like and then you will never regret it. (I'm going to try follow my own advice here myself and get a keyboard soon I hope)