Stihl are the best. Husquevarna gives you nearly the same quality at significantly lower cost. Stihl parts and service are only available (legally, that is) through Stihl authorized dealers, who don't compete with each other. Husquvarna parts are available everywhere including online and big box stores.
My advice is this: if you want to do EVERYTHING yourself, including chain sharpening, buy a huskie. If not, and you've got a nearby Stihl store, go visit it, and see if you like the people there and they have a good stock of parts and a good service desk. If they do, and you can afford it, buy a Stihl. You'll never regret it. If you have no local Stihl dealers or the locals are chumps, get a huskie.
Good chainsaws generally come in 3 flavors: pro, farmer, and homeowner. Pro models are super-light, super-powerful, and BLINDINGLY expensive. Do not buy unless you plan to earn your living with the saw! The farmer models (such as the Stihl Farm Boss) are for periodic, occasional heavy use. If you can afford one, they have good resale value and are extremely sturdy and powerful, though typically heavier than pro models. Then there's the "homeowner" grade, for people who will almost never use them. I have never used one, although I've pulled out my farmer grade saws many a time after someone with a homeowner grade saw found they'd left it full of gas for a year or more, leaving it useless until worked over by a maintenance pro. Homeowner saws are less expensive but have poor resale value and require just as much care as any other saw.
From the above, hopefully you've figured out that you can't store a saw for more than a month or two with gas in it and expect it to run right. Read and follow the manuals.
Size: a 16" saw will tackle anything less than 30" in diameter (although the big stuff requires planning and skill - again, read the manual) and you shouldn't use anything bigger than a 20" unless you're large, physically fit, experienced and skilled. I use a 20" routinely and although I have at least the first three of the preceding qualifications I would not attempt a 36" or 48" bar saw. I notice that many pros never use anything bigger than a 14" and do just fine by understanding the weight of and stresses on the timber they cut, so don't think bigger is necessarily better - it is mostly just harder to carry around.