I've got the FS 90 with handlebars and really like it. I mainly use it with the blade for cutting brush and it does very well. I've also got the FS 56 with lloop handle which I use strictly for trimming now. Before I got the FS 90, I did use the FS 56 for brush, but frankly it's undersized for that use if you've got a lot of ground to cover. I've only had a couple of occasions when I thought I needed anything larger than the FS90. And I have used the trimmer head on the FS 90 on heavy grass and it did fantastic, so if that's your main use, you'll be fine but probably will just want the loop handle model.
I'll second (or third or fourth....) only using good fuel. Absolutely avoid anything with any ethanol if you can, especially if you only use the machine intermittently-and that is coming from experience. Today I will only use 100 octane aviation fuel that I buy from a nearby private airport, and it only runs a little over $5 per gallon. I used to use a racing fuel, but frankly think it's overkill, and the lead in it may cause other problems from what I've read. By the way, we still are actively involved with a family farm and grow corn, so it hurts to recommend staying away from ethanol!
Good luck with your decision.