If you want it to last through hard service.
Light duty, Husqvarna (sp?), Poulan.. yes, I had a Poulan for 10 years before someone stole it...
Whatever you buy, it will have to be maintained properly. Always use clean fresh gas. Always keep it inside when not in use. Wipe it down every day after use, gease the front nose sprocket. Check the bar and chain oil often and keep those passages clean as possible.
VERY IMPORTANT! keep a sharp chain on it. A dull chain will pull too hard, makes the saw hard to control, will make the engine work much harder creating more wear, Will get very hot, reducing the life of the chain and the bar.. Lots of other reasons including quality of cuts. A dull blade can lead sideways and will bind up in the cut. This creates a dangerous situation.
Careful consideration and word of mouth will get you a good quality saw. Talk to the people who know, not the salesmen... but the small repair shops. Just stop by and ask. The worst they can do is not answer They will have a wealth of information and some honest opinions too. If you make friends with them now, they'll be easier to deal with later.