hmmm, that's a tough one. I really prefer the Mariner as a ship. Nicer suites, more dining options. But you'd miss out on the motu day at Tahaa, the atmosphere would be less Polynesian, no Gauguines, no watersports platform. But if all you're really concerned with is relaxing and enjoying the scenery, I'd pick the Mariner. 18 days sounds very good to me!
