Is Ducted fan or Propeller better to maximize forward speed of RC aircraft with a given power?
A propeller is better. Just look at any pylon race model to see what they use.
DKNguyen put it very nicely:

A duct doesn't beat a larger prop, it only helps make up for a smaller one.

You're right that &quot;EDFs induce a greater delta V on the air they move&quot; but EDF models generally use twice as much power as a similar aircraft with a prop. Moving half as much air twice as fast generates the same thrust but requires twice as much power. This is because momentum is mass x speed, but energy is mass x speed squared.
Ducts (and multi-blade propellers) are used when you don't have space for the right sized prop for the engine power.