I have an F150 with the sway built in and also have the WD with anti sway bar. I have experimented with this same question. I find if I am going on a short trip with no highway involved (which never happens unless I am taking the camper for service or to my local dump station) I don't need the sway bars. If I will be on the highway, my trucks anti-sway does pretty well, but the 2 together work great! So I use both.
2013 White Hawk 28 DSBH
2011 Jay Feather X23J (traded-in)
2011 Ford F-150 XLT w/towing package
RoadMaster Active Suspension
Reese Trunnion Bar WDH w/Dual Cam sway control