How much strimming do you want to do? If a lot, buy quality. I'd go for Stihl anytime, but pricey!
We have a Stihl one, and it just goes on and on. Can be a bit of a sod to start when cold, you have to learn a knack, but once you have it, they are a great bit of kit.
The other thing is, you can get a head that has 3 little white blades, that can fold in, and are held out by centrifugal force, rather than using they nylon line that seems to keep snapping, and lasts a lot longer as well.
As said, the auto feed can be more trouble than it is worth, when I had electric ones, I found that the friction of the unused line flexing one the spool, would get it hot enough to either weld it's self together, or weld to the spool, resulting in it not feeding, so you have to take it all apart to get it to wind out.
The last electric one I had was a Flymo, which fed a bit of line out every time you allowed it to run down in speed, and it had a little blade on the edge of the plastic case that trimmed it back to the right length, but not only did it have the same problem with the line welding to the metal bush it feeds off the spool through, it went through line at an alarming rate.
