I own the fs-91 and an fs-100rx and if I had to buy a string trimmer tomorrow it wouldn't be a stihl, or at least not either of the models I have. Read about issue's with any model you are considering, both of mine use the same carb (Zama) and they are crap. I just got done rebuilding the carb on the fs-100 a week ago and after reading around it looks to be a fairly common thing to have to deal with no matter how well you take care of them.
TBH, if I had to buy one tomorrow I would probably look into a Echo. I had a Husky years ago (before I knew any better about taking care of them) and wish I had it back again, lighter and did just as well for general work and the odd heavy cut. I will admit when in heavier stuff the stihl does great but I don't encounter that enough to constitute the additional weight. Put in plain words, I over bought on the trimmer and the added weight sucks when the torque isn't needed.
