I would recommend seeing if you could try each out before making an investment. I don't have experience with the SS/N19 scooters, but I think the trigger/handle mechanism would be a concern for me personally. There are also the Zuexo scooters, which I have heard many good things about.
1) Personally, I own a Sierra, and for me the choice was mainly between it and a Gavin, and I chose the Sierra for the weight. That being said, I (personally) believe that my Sierra is underpowered compared to some of the Gavins that are around, and have had some issues in very heavy currents with lots of gear.
2) There are a couple of reasonable deals on Gavins on eBay right now: http://shop.ebay.com/?_from=R40&_trk...All-Categories