Two philosophies of sound
Steinway (Hamburg) — a bright, powerful, singing tone; a projection built for the concert hall that carries beautifully into a large home. The action is famously immediate and rewarding to serious technique.
Bösendorfer (Vienna) — a darker, more overtone-rich sound; a warmer bass with famous depth (the Imperial 290 has extra sub-bass keys). Preferred by many pianists who play a Viennese, chamber or lieder repertoire.
Choose the room first
A parlour or salon grand from either atelier will sit well in most serious private rooms. A semi-concert or concert grand — 7 feet and above — needs volume, height and thoughtful acoustic treatment or it will simply be loud. See How to create a salon room at home.
Which pianist, honestly
A pianist who plays a lot of Beethoven, Brahms, Schubert, or contemporary chamber music will often be quietly happier at a Bösendorfer. A pianist who plays a broad repertoire — including Romantic virtuoso work, Rachmaninoff, or 20th-century concerto literature — often lives more comfortably at a Steinway.
Both instruments reward serious playing. Neither rewards being under-played. If the piano will mostly be closed, buy something smaller and simpler.
The other names that belong in this conversation
Fazioli, Bechstein, Blüthner, Grotrian-Steinweg, Mason & Hamlin. Any of these can be the right answer for the right pianist and the right room. See Best grand pianos for luxury interiors and Grand Pianos — the pillar.