I'm actually _not_ a pianist, which often surprises people! I did of course start on the piano, but all of my high level training has been on the organ, and the technique is different enough that I don't feel comfortable playing pianistic writing anymore. I can pretend to be a pianist when necessary and make it sound good enough, but a competent pianist would notice plenty of flaws.

I can't speak to PianoTeq, but I've used both GrandOrgue and Hauptwerk as mentioned in the sibling comment. As a mostly FOSS user, I'd like to be able to just use GrandOrgue, and the software itself would work well enough for that. However, the quality and diversity of sample sets (organs recorded, one pipe at a time, to simulate playing different organs around the world) is nowhere near that of Hauptwerk. I work in a very Anglican music program with an English leaning organ, so being able to simulate an English cathedral organ is important to me. The organs available for GrandOrgue tend to be more German Baroque leaning and much smaller. In an ideal world, you'd be able to buy the same Hauptwerk sets for GrandOrgue, but licensing prevents that.

I once worked on building a GrandOrgue file editor, but the specification is a nightmare. I'd like to finish that project someday though.

PianoTeq is considered the best piano sound. some even argue it is better than a real grand piano (it never goes out of tune) - but only if you have a great keyboard (several thousand dollars!), and sound system (more $$$) They are not known for organ sounds though.

If you want organ sound, then the software to look at is: grandorgue, Hauptwerk ($$$, but reported to be great), Aeolus (simulates pipe via math, the rest work on samples of real organs). There are probably others, but the above is what I see come up most often when this is discussed.