Well when it comes to accessory electronics, Smart-Fly is pretty tough to beat not only with what they offer but the CS is outstanding.
However for an optical kill switch, I prefer the ElectroDynamics unit since both pieces are contained in housings which makes 'em easier to mount/cushion without worrying about exposed circuit boards. While they are more money than some others on the market, they're about the same price as the Smart-Fly and the CS from ElectroDynamics has been excellent in my experience.
http://electrodynam.com/rc/EDR-107/index.shtml