I bought a pair of WP5's in 1995. While overall they are great speakers, they have an upper midrange that is troublesome. On most recordings they sounded great, but massed strings were always a problem. Any brightness in the recording was magnified. But with the right recording they were magic.
Over the years I finally decided I needed something a little more relaxed. In short, a speaker not quite as ruthlessly reveling as the WP5. I sold my WP5's and, like you, am looking to replace them.
I have it on good authority, a Wilson dealer, that the big jump was from the WP5 to the WP6. The WP6 toned down the brash upper midrange quite a bit. The WP7 and WP8 continued with the refinements, but these were smaller, more incremental steps.
The Sasha and Sophia 3 reflect the improvements implemented in the Maxx 3. The WP8 and Sophia 2 have the tweeter from the Maxx 2. Anyway, if history is any lesson the choice of the Sasha or Sophia 3 will depend on the size of your room and your need for very low bass. Chamber music in a room of 250 sf isn't likely to sound much different. But the Sasha would likely do better with Mahler in 500 sf room.
I'm considering the WP8 or the Sophia 3. The Sasha is out of my price range. A used pair of WP8's, while attractive, aren't that much less than the Sophia 3. To me, if the Sophia 3 really has a more refined sound I'd prefer that to the extra slam I'd get from the WP8.
I'll look forward to your comments about the latest generation of Wilson speakers. Since you and I were in the same boat for so long I'm very curious as to your thoughts.