Yamaha S03 is a synthesizer, not a digital piano.
Again, my suggestion is do a search on this topic as it has been covered before a few times.
We need to first get our definitions right so we are on the right page. In fact, different manufacturers have different definitions, so it makes things a little more complicated. But basically, digital pianos refer to electronic keyboards that simulate the piano as much as possible. It does not allow you to alter the sound envelope (like a synthesizer). It just plays back.
There are those meant for home (come with build-in speakers) and those that are meant for gigging (also called stage pianos, with no build-in speakers). Again, Roland and Yamaha's definition differs here.
OK. After getting that out of the way, you'll need to ask youself - do I need 76 or 88 keys, are the touch important, do I need to carry it around with me or will I use it only at home, do I need build-in speakers?
For home use - the best digital pianos are the Yamaha P series. Cream of the crop - P250. Closest touch to the real piano. For cheaper ones, P120. Also on the cheap side - Roland ep series (comes in 76 and 88 keys). Good sound but touch not as good as the Yamaha.
For gigging (stage pianos), Roland RD700 is the top - great touch and sounds and extremely ergonomic for the performing live keyboardist. Cheaper range - RD170.
Casio is cheap. Touch not as good, sound not as good. But if cheap is what you want, Casio is OK. But not good as an investment. I think most of them comes with the red light in the keys which is irritating.
Digital pianos are usually not cheap - min above $1000. You'll have to try around and see for yourself.
