I found seatpost racks that bolted on not to sway, the clip ons do a bit but not if you really crank that baby down. Personally I like the clip on as it allows me to leave the trunk bag on and just carry the whole works into the buildting with me without wearing out the velcro on the seatbag fasteners. If I used panniers it would definitely be a different story. I wouldn't use panniers with anything but a traditional bolt on rack.