I have actually used both!
I am very satisfied with the screw-in anchors. Bear in mind that you shouldn't just "get-em and crank-em in". I started out doing that, and the first set of anchors I bought inspired no confidence. So I set out to get the right kind.
I discussed it with a company that supplies construction contractors. They had some big ones that get used by utility companies. After looking through their books I selected a middle sized type. They screwed in easily, they passed a pull test, and they have absolutely not moved now matter how tightly I crank the turnbuckles on my cables. If you want a set of these, you will look for the ones with forged "knuckles", not just a welded loop.
As for duckbills, we used them successfully on a project at work. They are about as easy to install, however you can get into trouble putting a duckbill into hard earth, because the tamping rod isn't stiff enough. The duckbills are pounded into the earth with a rod, and the duckbill has a socket about 1/2" diameter. Well, you try hammering on a 1/2" rod that's 6 feet long. Talk about spaghetti. We ended up making a rod twice the diameter, and turning down a tip to fit the duckbill socket using a lathe. That rod was stiff enough to take the pounding of the duckbills into just about any earth for the rest of the project.
They are both good tools, but you must select carefully in either case. Read the product literature, talk to the supplier, and when you buy them, get spares. And test one!
