I had the 3909 kit and it is a fair improvement over the stock air cleaner. I have now installed the 3910 kit and it has a mildly thicker filter. I have not had any issues with knee placement or comfort. The 3909 kit will flow more than enough air for a mild build (like mine) but I had to have the thicker filter..... Good luck with either one you choose.
Remember, Ride Safe..
2008 FXSTC, V&H BSS, K&N AC kit, SE255 cams,
ThunderMax w/AT, Superbrace
Remember, Ride Safe...