I recently replaced the original Ford shocks on my 2012 E-350 (33,000 miles), with a set of Bilstein HD's. I could not be more pleased with the dramatic improvement in ride and handling. I have no idea what the Koni's cost but have a very hard time believing they could provide any improvement over what the Bilsteins have done.
The Bilsteins were $396, no tax or freight, to my door in 4 days from Florida to Tucson from Shock Warehouse. I spoke with Bilstein and Shock Warehouse and they both recommended that I go with the HD model as we usually run fairly close to our GVWR (11,500), and often tow a 960# utility trailer for my motorcycle.
As always.... Opinions and YMMV.
:C