Just read this and the other Gator forum to see what issues folks have had with their Gators. You should also visit the Ranger forums to see what issues people have had with the Polaris. Then compare the issues that both machines have. Neither is perfect so you'll just have to decide for yourself which is better. Keep in mind that few people sign into a forum and just start a thread to say their machine is running OK. Many folks only post when they have a problem so just because you see threads about problems it doesn't mean a machine is junk.
The Gator 550 did/do have a couple issues but Deere has been addressing them under warranty so knowing the maintenance history of the machine you are considering is important. The original engines often blew head gaskets. Deere installed a new type which largely resolved that problem. Then there is the drive clutch. It too was a problem and Deere replaced them under warranty. I don't own a Polaris and I don't visit their forum but my neighbor has a ATV/UTV repair business and the Polaris and their clutches are not free from problems either.