With the way tools are made now they are all probably coming off the same assembly line in China.
As to the two or four stroke...two strokes are lighter and simpler but require the gas and oil to be mixed. Four strokes are heavier and more cumbersome but the oil and gas doesn't need to be mixed. It does still need both, regardless of what some people think.
Another thing you might want to conisder is the convertable models. They can be changed to several different tools, such as weed trimmer, brush cutter, cultivator, hedge trimmer, edger, chain saw and etc. They do come in handy and with not buying a motor for each seperate tool, they can save you a lot of money.
We're on our second year with a Ryobi 4 stroke with a few adapt-it attachments. I've had a Homelite 2 stroke since 1992 that still runs pretty good, but the wife hated having to mix the fuel.