I looked at quite a few 4 stroke weed trimmers last year and decided on the Stihl, which is more of a "hybrid 4-stroke."
I've used it quite a bit and found that it has far less noise & vibration that the 2-stroke we still have. The one I decided on is one of the larger, more powerful trimmers and it makes quick work of even the tough stuff.
Where our smaller 2-stroke Stihl would bog down doesn't even phase the bigger 4-stroker.....
What sold me on the Stihl was that, dollar for dollar, I got more machine for the money. My wife's cousin owns a Tree Removal Business, her Dad a Lawn Service and while they don't use Stihl products exclusively, they do run 'em on a daily basis. When asked, they both told me I couldn't go wrong with a Stihl, but, then again, I've had a Stihl chainsaw & weed-trimmer for yeats so that really wasn't news to me.
Another that they recommended, and that I looked at, was the Shindiawa (sic) 4-stroke. The place I bought mine was large so they had all the brands I had reaseached, including Husqvarna, another top choice, which meant I was able to do an in person comparison before making my decision.
Having been down the Ryobi road before, more than once, I would never even think about buying another. Hard starts, stalling, you name it we've dealt with it.
The Pro's all use straight shafts. They are just easier to move around, get under/around bushes and gives you a better view of what you are doing.