Do you live on a 27 acre mountaintop compound, like most of the folks on this forum, and expect your weed whacker to deal with oak saplings, thorn thickets, and random intruders?
Or do you have a fairly standard suburban lot and need to trim the grass? Be realistic about your needs.
For the latter usage, pretty much any of the battery-powered models will do just fine. If the price stings, remember that you won't need to futz with crappy carburetors and crappy gas any more.
Also remember that the crappy cheap MTD 2-cycle weed whackers sold under 143 different brand names are worse than worthless. Same for the very cheapest machines sold by Stihl, Echo, Husqvarna. They're throwaway garbage.
If you really do need a gas-powered machine, you need to step up to a better level of machine, and these are not cheap.
If you've already bought into a particular tool battery ecosystem, then see if they make an option. I have a lot of Ryobi stuff, so I have a cheap Ryobi string trimmer and blower that use the same 18V batteries and work extremely well for my less than 1/2 acre in the city. I can do all the weed whackery and grass blowing needed with one charge of a 2AH battery. I've had these for about three years, and they're still going strong.
The very day I bought my battery-powered stuff, I set the 2-cycle garbage out at the curb. 2-cycle engines are officially banished from my garage henceforth, and good riddance to the stinky, delicate piles of ****.
There really is nothing like chucking in a battery and getting straight to work, quietly.
Or maybe you really do enjoy yanking starter strings with growing dread and frustration, endless futzing with teensy Chinese carburetors, and inhaling exhaust fumes on the rare occasions the damn things actually do run.