"Someone comment on the SR Tranquility?  I'm not having much luck finding one to try out.  I like stability over tracking and speed."

never paddled one, though I ran into a guy along a portage trail (below the Piceral R dam) who had one - stopped to chat a minute and I asked him about his boat - it was a SR Tranquility (an older boat, maybe there have been changes, I don't know) - he hated it and was ending his trip early - it tracked too hard, i.e., was too hard to turn as far as he was concerned.  I've not seen any glowing reviews of the boat, more on the negative or ho-hum kind of comments, but I've never made a big effort to find review either.

you might look on Paddling.net or CCR.com to see if there are any reviews there.

I've never paddled a Prism either, but it has the reputation of being stable - it is a mainstay rental boat for BWCA outfitters - logically, they will choose a solo boat that is easiest to get used to for people who have never paddled a solo before - might be worth looking at for you