I have to tackle a lot of very tough stuff, but do not like steel blades - they are unforgiving of mistakes and, I find, do not clear the cuttings as one moves along. I tried out an Oregon 2-line Jet-fit head with their (serrated) flexiblade and then bought a 4-line Jet-fit head as well for my larger strimmer!
Pros: The flexiblade line cuts everything. I seem to think comes in size range from 2.5mm to (more recently) 7.0mm. I've never had a line snap off, just wear away. The head takes any other type of line, but I seem to think 2.5mm is the minimum.
Cons: Flexiblade is not cheap p/metre and the way it is held by the Jet-fit head is very wasteful - about half of each cut length of line inserted into the head has to be thrown away - making it doubly expensive. If it does snatch, the line can sometimes pull back into the head, meaning no little stub-end to put your pliers around to pull out the "eroded" line: it won't always push through instead, which sometimes requires loosening of hex screws to free things up a bit so that the eroded line can be pushed through (rather than pulled). Also, don't risk using it "carefully" around shrubs/trees - if you must strim around these, use a much less aggressive type of line (and go easy on on the revs)!!
As I said, I have to tackle a lot of very tough stuff (e.g. thumb-thick brambles) - the Oregon head/flexiblade combo does the job (with just the occasional bit of faff if I really press my luck too far). There is even a 7mm line, but I've not tried it yet cos it only comes pre-cut and is, therefore, even more expensive. (There is a youtube vid' demonstrating the use of 7mm line.)
Tip: Whatever type of strimmer head and line you use, keep the line water-moistened in storage to avoid brittleness and reduce snap-offs.
Question: As I sometimes think a reliable auto-feed head would suit me for light duty strimming (can't be doing with bump-heads), any recommends please?