Synthogy is also a sample based instrument. you buy a sampler that plays 30 GB piano sounds. and that sound makes it expensive. it's a lot of work to record single tones in different dynamics and make a multi layered sampler instrument of it. I'm sure those pianos sound great.
on the other hand if you buy the Halion you probable won't get a lot of great sounds along with it. but the Halion reads the following formats: AKAI, E-MU, Roland, Kurzweil, GIGA, Kontakt, EXS24, SF2, LM4, LM4 MkII, REX, ZeroX BeatCreator, WAV, AIF, SD II (nur Mac), ISO & Nero Disc Image.
that means you can play pretty much every sound that has ever been made for samplers on your pc. there are thousends of cd-roms with with all kinds of sounds. from orchestra to drums to noise to vintage synths to drum computers...you name it.
have a look at this.
plus: you can create your own sounds with your wave files.