You can go here and listen to MP3 demos of every freakin' piano emulation on the face of the earth...well, every one except the Galaxy pianos, which aren't up yet (or maybe they're under a different name and I missed them.) You might consider downloading some of the mp3s so you can listen later and reflect on what you're hearing.
Digital Piano Shootout
All these pianos have their strengths, depending on the mix of the particular song. I have the Native Instruments Akoustic Piano, True Piano, and the Galaxy Steinway. The Galaxy Steinway is downloadable in 16 bit format, as are the other two Galaxy pianos, or you can buy all three in 24 bit on DVD. What's cool about the Galaxy playback engine is the extra stuff that comes with it - you can add a pad for new-agey stuff, you can morph the piano into a freaky techno thing - it's really quite awesome, and all for under $100 if you get only one of their pianos at the download site. Their mini-grand is absolutely luscious, in a dark, understated way. I'd get that one for solo stuff, or the other grand for a little more authority. Out of the three, the Steinway is the most transparent.
The True Piano is more of an emulation than a sample player, with a 60 meg downloadable 40-day demo that features one out of their four pianos. I'm using it in a busy mix where the piano needs to cut and the attack is perfect - you can hear the piano but it stays out of the way of the other instruments. The Galaxy is more of a ballad type of instrument with better fidelity but less presence - in other words, it doesn't cut through a busy mix as easily unless you sculpt the mix around the piano. I tried the PianoTeq, but found it similar to the True Piano in that it's also an emulation with a small footprint, but out of those two, I thought the True Piano was more realistic sounding. But people rave about the PianoTeq for playability and tweakability.
The NI Akoustic comes with four pianos, but they all sound kind of clunky in a mix, although they sound okay solo. Out of the four, the upright is passable, but rather generic sounding. Native Instruments also just released their funky upright piano collection for $279, but you can get a nice upright from Sampletekk for $49 called the Rain piano. The Sampletekk pianos require a playback engine, like Kontakt or Motu Mach 5, while all the others mentioned come with their own player.
For pop/rock stuff, the Sampletekk White Grand seems to be the winner. It's nice and bright when you play with authority, but can still be mellow if you take it easy on the attack.
Although the Synthology Ivory gets all the raves, it doesn't sound so great on that site linked above. There is something very strange going on with the midrange, which may or may not work depending on the context.
The Art Vista is great for character, and very tweakable with tone and compression built into the playback engine, but some say there are a few funky notes in the middle octaves which could be problematic if those notes are played repeatedly. Personally, I like that kind of character in a piano, and if I did more solo stuff, I'd probably get the Art Vista to complement the others I already have.
If you don't yet have a sample playback engine, Kontakt and Motu Mach 5 both come with nice big grand pianos, so that might be a good place to start, although if you're doing mainly rock stuff, the True Pianos would be perfect - and under $200 for four different pianos.