My 2005 CDI has a looped system. It is a well designed system. When I change the fuel filter about every 5000 miles with oil changes, I put a pail before the fuel switching valve and turn on the electric veg pump and it pumps grease and air out of the system, then reattach the hose. Mine has a 60 plus psi fuel pump. The loop is behind the veg fuel filter and fuel pump.

Mine also has three valves for fuel switching. One for fuel in and two for fuel return that way when I flip the switch on (they are all electric) the diesel fuel pump just returns to the diesel tank. When I flush the diesel the diesel goes in and the veg return switch is still on to keep from pushing veg in the lines into the diesel tank.

The loop eliminates a lot of hose and fittings for sure. I find it is important to use good fittings. I use push lock flare fittings and Pex line that I heat up and force the fittings into. This makes a great seal and no air gets into the system. And the price difference is negligible because I do not have any hose clamps in my system.