I tend to prefer the screw style, but most likely, that's out of force of habit.
In theory, the notched legs should be faster to deploy and level, as a guy can pick his inch by inch leg position, then level the rifle on the swivel. In practice, I generally work my set up to find level enough ground that I don't have to worry about having the legs uneven. Not to mention - it makes my brain itch to think about the torsion induced by having such flexible legs at uneven intervals.
I use 9-13's most of the time when I use spring pods, or the 13-27" for sitting. 6-9" gets you a little closer down on the bench or down on the deck, but in grass, it's often too low (any prone is often too low, for that matter). I know I have a 6-9" somewhere, can't think the last time I used it though.
Since you have a 6-9" notched, do you ever find it too short? Do you like the knobs or notches better, after having tried both? I like nobs better for myself because I'm used to them, but I'm prone to recommend notches for speed.
And like I said, I rarely find use for short bipods. If I'm on a bench or going flat, I usually use a ski-pod or a machine rest.