"Best synth for the money is a software synth"
FIrst about the hardware.
This should give you a broader perspective: http://www.synthzone.com/ubbs/Forum1/HTML/003296.html
I don't know what made you decide that Roland Phantom X is more advanced. but than again, appearance and big displays can be deceaving.
Number one thing to look for is software support, ease of use and expandability. Roland uses only sampling and sample playback synthesis. It is hardly more advanced than Triton or Motif.
The samples that you get with ROland Expansion cards aren't exactly fantastic and hard to use in the mix (actually this one goes for all ROland sounds). They sound alright by themselves but in the mix they basically "walk all over each other" if you know what I mean. If you don't, than do not find out the hard way!
My personal preference is the MOtif ES 8. Mostly for keyboard feel, plug-in boards that add polyphony and independant synthesis engines.
Here's my main point though. You have mentioned that you are new to this and that you used to working with computers. You have also mentioned that you find the quality of piano sounds very important. I think you should expand in that direction as in oppose to face eventuall disapointment of having to deal with limitations of Roland, Korg and Yamaha workstations.
Truth is, software synthesis and software sampling is far ahead of what any today's workstation has to offer. For example something like Steinberg's "The Grand" and Ilio/Spectrasonics "Ivory" deliver gigs and gigs of full length superb quality piano samples instead of some misarable 32 or so megs of short, looped and stretched piano sounds found on Roland Phantom X.
The pianos on Phantom X could sound very impressive at first, but once you get into playing and using tons of sustained notes and different velocity ranges you will find that the cross-fades are actually quite cruide and that the loop points are more obvious than they seem originally.
But really, instead of me trying to explain you the difference between something good and something crappy, check it out for yourself: http://www.ilio.com/synthogy/ivory/
The best thing about software instruments is price. They cost a fraction of what workstations go for.
My advice to you is this. Get a decent controller (something like Yamaha S90 will be perfect. It has the same keyboard feel as the MOtif ES8, good sounds that work well together and even takes the same plug-in boards as the Motif ES series). Than get some decent software plug-ins for your computer and rock on!
OF coarse if you prefer pain and suffering of using Roland Phantom X which will eventually seem like real nightmare GO AHEAD!
See, workstations have their place. SOme guys who play live obviously can't take their entire setup on the road and so they choose to get Phantoms and Tritons. They are cool synths, but really limited in comparison to software instruments. Again, if you want to learn it the hard way it is your choice.
Good luck on your decision. Hope you choose something that will meet your needs.