I don't have any real experience with your other choices, but I know a lot about Sierras. My husband and I have them, and many of my friends have them. They are light enough to shore dive with, and don't make the boat crew swear under their breath when you hand them up. I've gotten over 60 minutes of continuous burn time on mine (although not recently -- I almost NEVER run the scooter continuously on a dive). Our two have been relentlessly reliable, and it's not hard to do the maintenance on them (which would be important to you, because you're not where it's easy to send the thing back to get it done).
I wouldn't have much worry about buying a factory refurbished scooter. Private party used, not so much.
I wouldn't have much worry about buying a factory refurbished scooter. Private party used, not so much.