I've had a variety of gas string trimmers over the years and really liked Echo, but when it finally died on my last year, I decided to give cordless a try. I am a big fan of Makita cordless tools and leaned towards Makita for my trimmer choice. Amazon had a deal with 4 batteries and a duel battery charger with their string trimmer that was basically the cost of the batteries with the trimmer and charger added for free. It takes two batteries and so far, I've never run out of power with just the two batteries when doing my yard.
What has surprised me about switching to cordless is how quite it is, and how I still have that moment of hesitation about using it becase I go through the start up process in my head after all those years of dealing with gasoline powered trimmers. After that momentary pause, I'm then relieved to know that all I have to do it put two fresh batteries on it and get to work. The ease of using it is night and day better then any gas trimmer that I've ever owned.
I don't know who the best brand is, but I have had enough cordless tools to see how they change battery stiles, or voltage in their current line, and the older model batteries become harder to find. Makita has commmitted to their 18 volt line that they use in all their newest models. Instead of creating another battery, they just doubled the number of batteries they use for their bigger tools. I believe that Makita will continue to do this, and I will always have batteries to run my tools.