I was looking for an 'upgrade' from my S80 mostly out of curiosity because there isn't anything wrong with the Selmer S80.
Both mpcs are of a certain type, i.e long shank, small to medium chamber and this type is easy to keep in tune and they produce a smooth, homogenous sound.
The differences from the S80 that I noticed were that the Soloist has a thicker tone and is less projecting. It is also slightly harder to play and I had to move to a slightly softer reed, probably temporarily. I attribute this to the fact that the Soloist has a low baffle.
I recommend both the S80 and the Soloist for those who play in wind bands or chamber type ensembles or who do not need especially high volume or projection. (That said, the Soloist used to be popular with jazz players of the past) They are a very safe choice and there is a reason why they are both classic mouthpieces.