I have both. I like some things better about the Honda over the Sthil; i.e., no oil to mix, a lot better fuel economy, its longer allowing me to reach under bushes and whatnot, no where near as loud and in my opinion it does trim a bit better.
The Stihl on the other hand is lighter, has a better trimmer head (the Hondas head will for some reason unlock and throw the spool and everthing out if you get in some heavy grass/weeds), and it does a better job edging.
I do like the feel of the new 25cc honda as it is lighter (2 lbs less than the umk 431) but I haven't used one of those yet.