I went through this last year. I've bought all sorts of cheapy brand weed eaters and finally come to the realization that I wasn't saving money buying stuff that would just fall apart, or not work when I needed it.
I decided that I wanted either a Stihl or Echo. My local dealer sells both, so I asked him which he would recomend. He didn't hesitate, he said for weed eaters of the same price, you get more power with the Echo and a larger gear head.
On chain saws, he said Stihl was better, so I don't think he's pushing one brand over the other since I have two Stihl chainsaws that I bought from him.
One thing that I found that I highly recomend is the square shaped string. It cuts much faster and lasts allot longer then round string. It's more money, but not much, and the difference in performance is significant.
Eddie