I'm a newbie, but have done a lot of research in this area. Matter of fact, you could check out www.Birdforum.com
, Google, etc.
Here's how I would categorize them in terms of quality (scopes within each
category are not necessarily in descending order):
(1) Swarovski 80HD, Leitz, Zeiss Diascope, Kowa Prominar series, Nikon Fieldscope ED (scopes in this category range in price from $1000-$2600.)Why the great disparity in price? Because, among other things, the price increases with larger objective lens and special glass coating. Look for glass with ED (extra low dispersion) or fluorite crystal glass.
(2) Pentax PF ED, Leupold, Nikon Prostaff, Eagle Optics
Look for features such as: Waterproof, Fogproof, Nitrogen purged, ED glass and long eye relief. The bigger the objective lens the more light is theoretically captured; this is an advantage in poor lighting conditions or if you do digiscoping. Almost if not all of those in category (1) come with the eyepiece as extra. Angled eyepiece or straight eyepiece? It's a matter of preference and there are advantages to either.
Go to your camera store or birding store and try out the floor models. Also tag along on a birding trip and look through the scopes of fellow birders.