I have used a straight shaft Green machine that just keeps right on running. It is old and when it quits, I will get another brand because this company is out of business.

I also have a curved shaft Stihl which is a very good machine too.

After using both, I am going to stay with the straight shaft trimmers because my back hurts really bad after running the Stihl for even short periods of time.

It helps to extend the shoulder strap on the Stihland force myself to remain absolutely erect but still not as good as the straight shaft trimmer which never results in back pain.

I am also about 6-1.

Interesting - since I absolutely love my SRM-225. Even though at 5'-11", being 2" shorter than the OP should not result in such an uncomfortable experience. I wonder if angle orientation and arm extension has any impact.

I'm a righty, and fully extend my left arm on the grip handle, while loosely graping the throttle grip with my right hand - just letting the straight shaft pivot at it's center of balance, right arm controlling pivot direction. This orients the cutting head parallel with the ground while allowing me to stand erect.

I find this machine ergonomically perfect to my height - compared to the four prior machines - curved and straight, which I've used in the past.

Give a Redmax a try. I have one Redmax and two Stihls, they're exactly the same length but the Redmax is lighter and for some reason better balanced. Also I'm thinking you can move the handle upwards towards the engine some, this would in effect make the trimmer longer...