What model Stihl are you running because if I had to guess it's at best an FS90...
Huge difference between trimmers and brush cutters, most folks don't realize this but higher priced units are classified as brush cutters (it's in the horsepower or cubic size of the engine, forget which but that's what classifies it as either a trimmer or a brush cutter, whether it runs string or blades doesn't really matter much)... If you're running an underpowered unit it will only last so long, if price bothers you that's the cost of running one of these unit... Then it depends what you expect this machine to do and how good you are sourcing your own parts and doing your own repairs, otherwise I'm going to suggest sticking with the brand for which there exists dealer / service support somewhere close to you. And you can try a few 'other' brands but they all break down in time, some are better than others but that's from one unit to the next, not that one brand is but so much better... They all require some sort of maintenance at some stage, I can go a year or two sometimes with just the basics but sooner or later it takes a complete tear-down and rebuild (not including the block itself, that stays intact) usually they don't go much more than 4-5 years on one spell (air filters and spark plugs aside).
At some point brand loyalty spells out parts interchange, cross-referencing and mechanical expertise, if you stuck to the same brand for many years you might know right away what is wrong with it and where to get the part and how much it costs, for example... So pick a brand and stick with it, knowing what makes one tick is the secret, as for top of the line brands... Definitely stay away from Troy bilt and any of those other economy priced brands, Stihl makes an excellent product, there are many others like it but again you want top of the line for $300? Not going to happen, for a top of the line trimmer you're into brush cutters and these generally run $3-$400 maybe even $5...
You can try Echo, Husqvarna, Kawasaki, Toro, can't recall what else but again it's all the same just looks and how they're built.
Ultimately you have to decide not so much which brand per se, but which one you want to stick with because it takes 5-8 but really 10-14 years of running one brand machine and one brand only before I could think I knew what I was doing.