Hi Laurence. It will always depend on the room layout.
In my room 7.1 would have the side surrounds very close to the listener, so I went with a 5.1 and 5 identical Bookshelf speakers. Those can be placed perfectly at equal distances with angles following Dolby specs. But you need to sit in the middle to get the full enveloping sound.
Little more to the left or right and the sound bubble collapses.
With the TriAxial speakers, you can move around the couch without the sound changing too much. The sweet spot is much wider. But this is for my 270 square feet / 25m² room and 4 seater couch.