We've operated string cutters on trails for many, many years each summer and here is a few things we have learned. We must operate them at max speed or they don't cut worth a hoot. If your not working among buildings or plants, you hope to not hurt, larger diameter string is better. We've ran .155 string for years but are now trying to upgrade to .180 that's recently came out. Nonetheless, some stringy weeds are he&^$ll to cut and will wrap up on your head. Dry conditions will reduce that and as others have suggested, more cuts will reduce but not eliminate that grief. Some plants are just harder to cut!
I should add that we've used every steel blade made and still have them here today. We went back to heavy string as mentioned above.