This post, parametrised to 'how does piano x compare with pianos p,q,r and z' ought to become a FAQ, I think.
My 10c worth.
(a) All the modelled pianos sound a little 'plasticky' to my ears, as if the piano itself wasn't made out of real wood. Hard to exactly explain but I think the little thumps and resonances and so on from a real piano are an important part of what makes it sound 'real'. Unfortunately, captured in a sample set, these quirks can become irritating since most sample sets don't have 'round robin' samples and therefore although there are multiple velocity levels, each repeated note is essentially the same note. Unfortunately, capturing a sampled piano with, say, 5 round-robin samples per note X 16 velocity levels X 88 keys is one heck of a sample set, and so although in these days of multi-gigabyte machines, that's not a problem, what IS a problem is the huge load time, unless using disk streaming, and the problem with THAT is that it can be, to say the least, somewhat quirky and particularly under high polyphony loads. And the problem with pianos, is that the player can make demands, with rapid passages and block chords, that make direct from disk streaming quite demanding, although it certainly can be done.
(b) If you just want a piano part to blend with a mix, then any of the modelled pianos are fine, and they have the great advantage of loading extremely quickly. If you are playing an exposed part then you may either want some 'character' to the sound e.g sometimes the intimacy and slight intonation errors of an upright piano add immense charm. Or if you are a classical player and want to replicate a grand piano, then you will certainly be looking for authenticity. I'm not absolutely convinced that all the work that's been done to add sympathetic resonance and pedal release samples in sample sets actually works that well, because in most cases, as I said, its the same sample being played for every note. This actually ends up sounding more fake than not having the effect at all, in my opinion.
(c) If you're not sure and you're on a budget, my strong recommendation is to go to
http://www.sampletekk.com...ts.php?cat=44&pg=2 and look at these low-cost PMI sample sets. They SHOULD work with the free Kontakt 4 player, I think - although I am still using the very old Kompakt sample player from NI with them. The Bosendorfer 290 is my 'go to' piano but each of them has a unique sound. At 50 bucks each they are a good way of acquiring a decent set of well-sampled pianos. Also you get the WAV files for each sample,they're not protected in any way so you can do all sorts of interesting things with the raw samples, should you be so inclined.
sorry - forgot to add that you can reduce load time with these large PMI sample sets by simply using the (free) R8Brain sample-rate convertor software to resample at 44.1KHz/16 bits. This considerably reduces the sample set size. However the audible effect is insignificant. I simply cannot hear the difference, and even trying the old 'phase invert' trick and recording a note at 24 bit resolution and then cancelling it out with a 16 bit recorded copy, leaves the difference signal somewhere WAY down in the noise level. Sampled instruments don't need 24 bit resolution; that's only important when RECORDING so you get the headroom, but if the samples are already recorded, headroom isn't an issue, they've already been normalised.