A solid steel shaft is what Shindaiwa uses on all its trimmers. Their shafts also have hardened splined ends. Probably less chance of braking a solid shaft. I am also told that you shouldn't use a saw blade on a trimmer with a flex shaft.
I have used both types on many different trimmers over the years and honestly don't see any difference. We have used attachments not made for the specific trimmers, blades to cut large saplings and zero maintenance as far as greasing the gear head with no problems, ever. So get whatever trimmer has the power that you need and don't really worry about anything else. I will caution about stihl's. They are fairly tricky to start even when warm. They make great blowers but haven't gotten the trimmers right.
i wrok on my own stuff on am running a 5 year old fs 74 , the only thing is with a soilid shaft you can run metal blades for cutting woody type brush, where the flex saft won't hold up as well, also if you should ever get your shaft tube bent (and i have done this) you can unbend the shaft and keep on trimming i ran mine kinked for about two weeks until i finnally changed it , a solid shaft would be hard to get by doing that.
Solid shaft are a lot better because you don't have to maintain them. Flex shafts have to be pulled out and lubed occasionally. Also manufactures who do solid shafts alot of times will warranty the shaft for life, because you can't ruin them.
Typically the solid shafts are found in the straight shaft trimmers. Very few straight shafted trimmers use flexable shafts. So the difference is the trimmer itself the straight shaft is a much more balanced trimmer then the curved shaft also it is more out in front of you and easier to get in under trees bushes ect. As far as performance I don't think that there is a lot of difference.
I HAVE USED BOTH. I HAVE A WEED-EATER FEATHER-LITE CURVED SHAFT WITH A FLEX CABLE AND IT HAS A LOT OF VIBRATION. THIS YEAR I BOUGHT A HUSKY 322L. IT HAS A STRAIGHT SHAFT WITH A SOLID SHAFT AND ONLY WEIGHS IN A 8.4 LBS. ALMOST NO VIBRATION. WOULD BUY ANOTHER ONE. STARTS ON SECOND PULL AND RUNS GREAT. PRICED UNDER $300.00.
TRY THEM BOTH OUT FOR YOURSELF AND GO FROM THERE.