From the bench and from a level platform shooting prone I like the 6-9 inch model the best because it's what is most comfortable to me and is the most rigid. If I was shooting just off a bench I'd go with the non swiveling model because you'd have time to set up and adjust the legs so that your rifle is not canted. However in the field to set that up is a pain in the butt (time consuming) and the swiveling model lets you just tilt the rifle to level it out.
All that being said.... I've found when I hunt with my bipod I usually end up having grass in the way or an uphill or downhill shot and I actually use a 13-27 swiveling model when in the field just to be more versatile. It lets me shoot sitting down if I need to and I've just found it to be the most useful.
So my solution was to buy both
I use the 6-9 inch swiveling model from Shooters Ridge when I shoot off the bench or when I know it's height won't be a disadvantage but I grab my tall Harris when I head off into the hills hunting because I don't know where I'll end up shooting from.