Rod, I've never been on the Silver Spirit, only the Shadow and Whisper, which are similar in size to the Navigator. (Cloud and Wind are smaller, and different) But both dougburns and colonelwes have been on Spirit, (on the TA in January) and both wrote reviews along the way, I believe here, but also on CC. Interesting reading if you can them on both lct and cc with a search.
From my two cruises, I thought the ships were so similar in every way to Regent that I wished if R had to merge with someone, it would have been with SS. However, the Carson family chose to sell to Apollo, which is associated with both Oceania and I believe, in a smaller way with NCL.
Perhaps it was the company I had (my daughter on the Shadow, all the BK'ers on CAMEL on the Whisper) but I would not hesitate to book with SS again, given my good experiences, with the right price and itinerary.