Assuming you are getting one saw and keeping/using it for a lifetime, I'd get a saw balanced between size and power.
I'd also get a pro saw over a home model, but that's just an opinion as some of the home saws are very robust. As a general rule, for a given horsepower, pro saws are a pound to two lighter and usually incorporate magnesium or other metals vs the plastics used in home models. The downsides are more expensive and often they are tuned for power to come thru at higher revs which means they are a bit less snmooth across the power band.
For me, I would be looking for a 55 to 65 cc saw as this is big enough to handle 18"-20" bars yet small enough to not kill you with weight while trimming. I'd be looking for a powerhead weight of 10-13 lb and horsepower rating in the 3.5 to 4.5 range. It's all just tradeoffs w/ weight being the most important for long days of cutting. Saws I'd go for:
Stihl: New/Pro: 362, Late Model/Pro: 361, 036, 034. Home: 250, 270, 290.
Husqvarna: New/Pro: 346, 353, 357, 359, 365, 555, 562. Late Model/Pro: 351, 362, 254, 257, 261, 262. Home: 350, 455, 460.
Jonsered: New/Pro: 2252, 2256, 2259, 2265. Late Model/Pro: 2152, 2156, 2159, 2165, 2065.
Dolmar: New/Pro: PS-5105
Unless you do your own repairs, dealer service should figure into your brand decision. Also a great way to go is to watch ebay/craigslist for one of these in good used condition. They can often be had for under $250 if you watch persistently and give it a month or so.
Nothing wrong w/ Echo, Poulan, Sears, Husqvarna 100/200 series, etc., they are just geared strictly to the consumer and are made with a balance between low price/min performance level. Almost any saw or saw brand you pick will last a few acres, it may not however stand up to the rigors of time. It's all just a big trade off for the most part