Well...
I acquired a Stihl fs560 for the needs of cutting grass in 10 acres of farmland. Going for such a powerful machine made my life easier but I encountered a serious problem. Due to the power of the Stihl, I constantly needed new trimmer line. After countless trials, I ended up with the Oregon 4mm square trimmer line. What this specific head offers me is the ability to have 4mm trimmer line folded, essentially giving me 8 trimmer lines on the cutting head.

I went down to 3.75mm from 4mm because there is a downside to the whole situation. When you need to change the trimmer line, it becomes very difficult.
In terms of durability, it has lasted me 3 years with heavy use!