Yamaha piano sounds are very bright, perfect for pop, rock, country music, etc. Not good for orchestral type classical or jazz. The Motif ES has somewhere on the order of 170 megs of ROM. The Fantom has 128. The Motif spends a lot more wave rom on guitars, basses, etc. They call it megavoices. But, with Roland, you must not forget the SRX expansions.
SRX-02 Concert Piano. adds 64 megs of a very nice piano. The piano in the Fantom X is already superb, but the SRX gives you still another choice.
SRX-04. Symphonique strings. 64 megs of lush string sections, complete with various articulations. No solo strings on this board.
SRX-10 Dynamic Brass. 64 megs of trumpets, trombones, french horns, and tuba. With many articulations. No solo instruments; only sections, except for tuba. that is solo
SRX-06, Complete Orchestra. 64 megs for everything else you need. Solo strings, solo brass, more ensemble strings, woodwinds, (including bass clarinet; how many other keyboards have bass clarinet?)orchestral percussion, harps, harpsichord, and vocal choirs.
Total, 384 megs of sounds; almost all completely taylored to your piano/orchestral needs.
Not to mention, you can expand it further with your own samples, up to another 500 megs; though this will take more investment, both time and money.
Good orchestral sounds are the hardest thing for keyboard manufacturers to produce. There are so many nuances of acoustic instruments so it's hard to cram it all into a keyboard. This is where Roland excells; they make expansion boards in great variety so you can customize your board for the genres of music you produce. But you cannot be afraid of spending money, or you'll end up with cheesy sounds that will leave you unsatisfied every time.