A few things to consider, brand doesn't matter near as much as a good dealer if you don't know much about saws and want to learn to work on them. Visit some dealers, handle the saws, see how they feel to you in your hands, too heavy, too bulky, does it balance good?
How does the dealer treat you, is he running off to deal with something else and ignoring you or attentive and answering questions? I've got three Stihl dealers within 10 miles, I will only visit one of them, the others are jerks! I don't care if you buy the best saw on the market, if the dealer is a jerk it's not worth owning! I choose a dealer, then a saw from them.
Used, there are more options than you can count, but again, you will need parts some point and time and it's nice to run down and grab a chain, air filter etc, from a dealer and cut again rather than wait two weeks for an online purchase!
What budget are you trying to stay at? Look at Echo if you are on the cheap, I know I bad mouth them, but they really aren't that bad of a saw for the price.
Does anyone know anything about the "Poulan Pro 20" 50cc Gas Chain Saws PP5020"
My dad has one, I don't care for it, but he likes it. First thing I would do is take the 20" bar off and get an 18" for it. Look around, you can find them refurbed for around $100. Taken care of you should get two or three seasons out of it, but they don't seem very durable in the long haul.
For a one saw plan, I'd get at least 60cc, 50 should work for you, but I like to go one size bigger, if you can, for that "one tree" that you need more grunt or the ability for a longer bar to fell.
Two saws are nice to have, get one pinched and you can cut the other one out, been there more times than I care to count:hmm3grin2orange: I cut Oak 100 miles from home, I take at least three saws with me, just because!
So to surmise, find your budget, find a dealer that treats you right, handle the saws in your price range, find the one that fits you and if you can, is one size bigger than you need then buy a cheap refurb Polaun for a back up saw. (or watch Craigslist for a used deal)