I worked in a mower shop for 2 years, the number one problem that we saw was old/bad gas in carburetors.
On the fuel front check your area to see if anyone sells gas without ethanol. Here it runs a little more than premium. If you can get it use that for all your small engine needs.
Regardless of using unethanoled or stabilized fuel. If you are going to use your trimmer/mower weekly you can get by with leaving the fuel in the tank. If you are not going to use it for a month, then drain the fuel out, or best bet is run it out of fuel. On you trimmer once it has run dry, then be sure to pump the bulb a few times to blow any residual out of the carb. Be absolutely sure to drain well at the end of the year.
If you are not opposed to a electric, Core trimmers are really good. They are about $100 more than a Echo, but there is no fuel to burn so you will make up the cost on the back end. These have a ton of Torque, and can go quite a while on a charge. These are not your black and decker/toro rechargeable. The shaft and lower end was designed by Echo. The Battery is in the top end and a Circuit board motor is in the lower. We never had a unhappy customer at the shop I worked at who purchased a Core trimmer people actually raved about how happy they were with them.
As far as brands to buy. Both Echo and Sthil are good brands. From a mechanic perspective a Echo is generally quicker and easier to work on, as the Carb is accessible without removing a bunch of covers. I also found Stihl more difficult to work on, as they tend to be a bit over designed.
IMO spend the extra few bucks for a straight shaft, your lower back will thank you. Try one out in the store, the curved shaft you seem to bend over a good bit more to use it, and the straight shaft gets under things much better.
For me I would talk to local lawn mowing services, see which brands they use, and how the service is. In my area most pros use Echo, In other areas its Sthil, For a pro it probably comes down to Service from the shop as they are much harder on equipment. Hopefully you wont ever need service, but in case you do you want to be going to the best place.