What kind of sounds do you want? Certain manufacturers do certain things well.
I think Yamaha does nice "realistic" sounds and synth lead patches in their latest soundset ... does
the S03 use the same sounds as the S90 or Motif? That may be a nice bet for that ...
Roland's sounds tend to be a little "hyped," like they take a sound and either record the sample or process after the fact to bring out something that's the essence of that sound ... for example, a sax sound might not be as full as on a Kurweil or Yamaha, but it will have the bite and reediness really accented ... they process it ... that could either be easier for you, or limiting, depending on what you want to do ...
I think the Alesis sounds are somewhat "plasticky" ... if you do techno/electronica/house/trance this could be EXACTLY what you want. If you do more organic roots-rock, you may have better options ...
Korg has some nice sounds, "processed" sounding in a different way from Roland.
I don't have experience with Emu to comment ...
Have you gone to your local music store, requested a pair of heaphones, and just played several different boards yet? That's a great way to get familiar with the "sound culture" of each manufacturer. Different sound-set personalities definitely come out. Then it's a matter of weighing those sounds with editing features and price to come up with a compromise that works for you.
Sorry for the long non-answer, but more information goes into the decision than we have available. Good luck!