I haven't used the hedge trimmer so I can't really compare the two. I used the scythe on the bushes at a neighbor's place and it worked real well. I did do quite a bit searching and reading before the purchase and from what I read the blades are very similar and differ in length. The gear box of the power sythe is supposed to be a bit more robust. I visited a real hardware store nearby and mentioned that to their Stihl guru. He said he thought the gears were the same and it bugged him enough to go have a look at the part numbers. He called me later with an answer to another question I had and confirmed there was a difference between the gears of the trimmer vs. the scythe. The scythe has a stainless bottom cover that serves as a sort of skid plate because they expect it to be drug along the ground.
The scythe performs best on anything heavier than lawn grass. It worked OK on grass but the old Stihl string trimmer is a lot faster on pure grass. I can manage to use the string trimmer along the house and sidewalks without getting all green.
It really shines on the creek banks and cutting the fencerow and tree branches under control that want to scrape the tractor and me. The adjustable angle of the head is very useful. The creek banks have been difficult to deal with ever since we built here in '89. Regardless of how carefully I start out with a string trimmer it always reaches the point where I end up proceeding with reckless abandon and getting so miserably covered with plant bits and juice that I need to hose off my glasses and face afterward. With the scythe, the cut plants simply fall over. My clothing, glasses and face are as clean when the trimming is finished as they were when I started. Since we have poison ivy lurking here and there, that's major for me.
Mine is a 94R and has plenty of power. It will saw through tree branches to about 3/4" without too much effort.