Harness and bicycle handles on the cutter is the way to go with a larger cutter. With the harness you don't have to hold the machine up, just aim it. And the wide handles allow better control of the machine. Stihl sells some nice harnesses. With the harness I can go for hours before I'm tired.
Sadly, we disagree. The only gain to be had with a harness is that it puts some of the weight from your arms onto your shoulders.
Apart from that, they prevent the operators from moving the cutter forward or backwards. They simply don't allow that kind of movement, so the operator ends up walking inches forward or backward to control what they hit. The operator cannot really control the up & down motion, except as it pushes against the harness attachment point. Basically, you are so strapped into the contraption you have no freedom of movement except a really lame arc of horizontal travel.
I had a guy fall off a very steep hill once, brush-cutting with a harnessed-trimmer. He tumbled and rolled about 40 feet down a 1:1 slope, getting the crap beat out of him by the trimmer, on account of the fact that he couldn't toss it aside.
That's another reason for not liking them.
The loop handle trimmers put all the weight of the machine and it's momentum on the operators two hands. After that, there is total freedom of operation.