Does a windmilling propeller create more drag than a stopped propeller in an engine out scenario?
I would not follow the advice in the forum. While the reasoning that a windmilling prop does create more drag is sound, I have seen no empirical evidence that says how much it actually translates to in Feet Per Minute. The only studies I have seen have been inconclusive on the subject, and say there's a number of factors that you can't really control in that situation. The prop isn't that big compared to the airplane itself, so if a spinning one has 30% more drag (just a guess, as I said I've seen no actual numbers) it isn't going to make you plummet out of the sky.

What I would recommend is following the engine failure procedure in the POH of your particular aircraft. The advice to slow the airplane to try and get the prop to stop has a number of flaws:


Distraction from critical tasks: Slowing the airplane until the prop stops is taking your focus away from finding a field to land in, ensuring your passengers are briefed and strapped in, making a mayday call and attempting to restart the engine. You'll be busy enough! 
Low airspeed danger: Slowing enough to stop the prop may require a very low airspeed, and you could have a stall/spin excursion, which is far more dangerous than a glide landing
Drag curve: flying very slow to get the prop to stop will put you below best glide speed, which is where you get your best glide ratio. Below best glide speed you start to develop a high rate of descent, so by the time you get the prop stopped you have lost hundreds of extra feet. You could lose more altitude than you would potentially save, and waste valuable time doing it
The chances are good that you couldn't change the prop behavior anyway: if your crankshaft has broken the prop's going to spin no matter what you do, if you've thrown a rod it's going to stop and you couldn't get it to windmill if you flew at mach 1


Doing some math puts this into perspective. Say an airplane's engine-out rate of descent with a spinning prop is 1000, and with a still prop it's 900 fpm. In a glide from 3000ft that buys you an extra 20 seconds, but it probably takes a lot more than 20 seconds to slow enough to stop the prop and then regain a stable airspeed. 

Flying very slow to stop the prop is a potentially dangerous distraction, just set best glide speed and if the prop stops its a bonus.