I have found that the SMALL dog classic long has very little tension. I also use the flexi with their harnesses when I let them out to the yard if they are already wearing them, I just don't fuss with harnesses when they have to run out and pee for 30 seconds. It's easier to attach it to the collar. You just CAN'T use the brake to reel your dog in like some people do. I *call* them back inside.
I also should point out that Loki is 12 pounds and Sammy is 6 pounds. I'm not sure how that impacts my decision. I know those of you with tiny ones leave those harness vests on all day, but we like naked dogs and nylon harnesses