There's plenty of opinions here, and I'm going to add some more.
If you're only going to use the whipper snipper for light work, then go for a cheapie and chuck it when it's knackered. If you want it to do a bit more hard yakka or get something that'll last, then maybe look into a more durable beastie. It might be better to fork over a bit of extra cash to get something that'll last, or can be repaired.
I have a few acres and really appreciate the guts of my Stihl brushcutter - I also use weed killer spray around fences and driveway. I'll be buggered if I'm going to trim a 50m driveway and over 400m of fence line perimeter every week through spring. Look at a straight shaft - there's less wear and lube required to maintain the flex shaft in the longer term. Also a straight shaft often allows you a wider range of heads. If you have a really tough job you can add a rubber/plastic tipped head, or a metal plate jobbie. (Just be careful using metal plates - they can be deadly and also create sparks off stones and in summer it could lead to fires)

I usually use a line, but for some occasions I've swapped to a metal blade. It truly is amazing what they'll cut through.
If you are going to turn your trimmer to the side (say to do edging) it's better not to go for a 4 stroke. Having oil in a sump makes it a design problem to properly lube the piston/cylinder. It can also lead to oil foaming thereby killing the oil circulation. To my mind the good old 2 stroke mix keeps the lube no matter the angle of the trimmer.

And if you have the time to look, there's often sales, or field days that you can go to and get a bargain llike shrek4 did. If you are buying a few items from a dealer, often you can haggle them down, or maybe get something thrown in like ear protection, face guard, extra bottles of lube, spare lines, etc.
Good luck sorting through all the opinions!!