A few points:
1) Unless you're an enthusiast who knows how to work on saws, your dealer is the most important consideration - even more than the saw. Pick a good dealer, and get whatever he/she sells and services.
2) There are two basic forms of saw construction, identified by how the crankcase is made: vertically split ("pro" construction, usually of magnesium but can be aluminum); and horizontal split ("homeowner" or "occasional use", also called "clamshell", aluminum but increasingly plastic.) Pro saws will have a better power-to-weight ratio, be a lot tougher, and are easier to rebuild to original specs. They're also a whole lot more expensive.
Clamshell saws are usually of lower power, can't be rebuilt as easily (and are usually not worth repairing), and cost much less.
Here's my personal opinion: a clamshell saw is a clamshell saw is a clamshell saw. A clamshell saw from Stihl is no better than one from Husqvarna or Echo, and any brand differential exists only in marketing. If you're going to buy a low end Stihl, for instance, you'll pay a lot more for the orange plastic but will not be getting a better saw than anything else. Again, a clamshell saw is a clamshell saw is a clamshell saw. It just doesn't matter a whole lot.
If you're going to buy a pro level saw, you have a lot of options and here is where you have decisions to make. Stihl's pro saws, on which they made their reputation (and of which their clamshell saws ride the coattails), are good choices; so are Husqvarnas. In my experience, the Stihls will be like tractors and the Huskys (this goes for their badge-engineered Jonsered cousins too) are the dragsters. That's changing a bit as Stihl introduces new models with higher chain speeds, but overall the Husky will usually cut faster than the Stihl. Both have good dealer networks and parts availability.
Echo has a couple of pro-style saws, but tend to be older designs that produce less power and have less effective anti-vibration mechanisms than their competition. Good longevity, as you'd expect from a design that's not being pushed to its limits. There is a virtue there, if it matches your needs.
Dolmar is a German brand that is probably the sports car of the saw world: high chain speeds, high power-to-weight ratios, with a slightly higher rebuild cycle because of it. Mine are extremely cold-blooded and take forever to warm up, but when they get going they cut like nothing else. As someone mentioned, the Makitas at HD are rebadged Dolmars and are great bargains. Unless you have a servicing dealer nearby, you'll need to learn how to do your own work.
Solo is another German saw, and one of the few that will sell any and all parts to the end user from their own website. Good quality, and if you like doing your own repairs (or simply have no saw dealers of any kind nearby) they might make a good choice. They are poorly tuned from the factory, so you need to know how to adjust the carb.
Redmax saws from Japan are little known, but the ones I've run have good pro-quality construction (I don't think they make any clamshell saws) at a decent price. My dealer tells me that he's had very little problem with them in long term use with the arborists and landscapers in town. (He's also a Husky dealer, and his extended family owns a couple of Stihl dealerships where he learned the trade. I trust his opinion.) If I needed a new saw tomorrow, they'd be on my list of candidates.
3) As to avgas: only if your carb is tuned for it, and you never run regular gas in it while so tuned. If not, just buy good premium gas with no ethanol and use quality oil.
