The D-10 won't provide as good sound quality as the X5. This is simply because the Korg X5 is a much newer model and has much higher quality samples & effects. The D10 isn't really even a sample based synth like the X5. It has waveform partials which were used for the attack phase,loop phase & drums and used those in combination with the onboard synth engine. Compared to the X5 the D10 sounds muddier & noisier. This wasn't a fault at the time as all early digital waves synths (including the Korg M1) exhibit the same types of problems, it was the state of the technology at that time.

It was a great keyboard when it came out, but I tend to think that buying one now would only really be a consideration if the price was low enough ie. $250 or less, otherwise I'd look around for a more modern Roland keyboard such as a JV30 or JV80 which probably won't cost much more but will sound better than the D10.