Given enough time ,in the sun's UV rays, either will break down. My vote would go for a high quality urethane paint finish though. The paint companies have done a lot of development to make these finishes standup on auto aplications where most vehicles spend their time outdoors. High end clearcoats have UV inhibitors in their composition sort of like a sun screen. Gelcoat is old polyester technology but does have the advantage of being thicker, perhaps alowing deeper scratches to be sanded out & buffed. My own personal experiance has been that paint is also easier to apply,repair & spot in, provideing its not a custom candy job. I'm in the auto body bussiness so I may be biased but that's my $.02