We use the Flexi 1 Classic LONG that is for 26lbs but goes 26 feet. I DO NOT walk my dogs on their flexis and people who do and don't lock them drive me crazy. "No, my 6lb dog does not want your 60lb dog's nose up her butt, no matter how friendly he is." Anyway, we use them because we do not have a fence and they are trained to go straight out to the yard and back. We need the 26 ft one because the 16ft one barely clears the sidewalk and they like more room to sniff out their spot.
I find that the 26 ft one has less tension than the 16 ft one, and has a slightly bigger handle which is easier to hold.