My thoughts are that if your choice is between a 15 year old raft, that has a few good years left, and a new raft, that while it does not have the "proven" record of the old one, should last at least as long as the "few good years" of the old one, I would take the new one.
I went through the same mental game over the winter when I was deciding on what raft to get. I ended up getting the Saturn. The thread that was listed earlier details why.
... and since I said the "S" word let the flames begin....