Salt water... Fresh water... all the same when choosing your mask. Mostly I'd consider the parts of the mask that are important to you. Purge or non-purge? The purge is a nice convenience. I like to compare it to power steering in a car. Most of our grandparents and many of our parents drove without it for decades and thought nothing of it. Today we'd not even consider buying a car without it. Do we NEED it? No. Is it a convenience? Yes. Basically it allows you to clear your mask simply by exhaling through your nose as you dive, without touching your mask. For me as an instructor and an underwater photographer, I love the fact that I don't have to fiddle with my mask.
Next, low volume without side windows or higher volume with side windows? Side windows were originally put in masks way back in the "dark ages" of black rubber skirts. The idea was basically to allow light into the mask and get rid of some of the tunnel vision effect. They were HUGE and heavy. Some had metal frames with stainless steel screws holding them together. They weren't selling all that well until some slick sales rep starting spouting off about "improved peripheral vision". Then all of a sudden divers were lining up for the kool aid.
Basically, if you hold a 1 or 2 window mask up, side by side, to an average mask with side windows, buckle to buckle as it'd sit on your face, you'll find that most low volume mask's lenses stop at about the same point the side windows start on the higher volume mask. Basically your peripheral vision is about the same. It pretty much comes down to comfort and impression. If you like the feel and view through one, get it. If not, get the other. There realy isn't a "right or wrong".
The strap really isn't a huge consideration exept maybe with something like the XS Scuba M-line series of masks such as the
Z Duo . A strap such as this has a pad that helps keep the mask from tangling in your hair when not using a hood, is a great deal thicker than the average strap so won't break as easily (not that I've ever broken a mask strap myself) and allows the diver to adjust the mask while wearing it as opposed to having to take it off and tighten/loosen the mask then try it on for fit.
Another thought is whether or not you require optical lenses in your mask. Drop-in lenses can correct for the magnification in your prescription or give you "guage readers" on the bottom portion of your lens. The more common
masks with optics use a common shaped lens that can be swapped out with the stock, clear lenses in a matter of minutes. If you have special needs in your prescription, you can also have custom lenses ground by an optician.
These days with most masks using some form of crystal or surgical grade silicone it is easier than ever to get a mask to fit one's face. Myself, I can get just about any random mask to fit comfortably and reasonably well. That wasn't always the case. When I first learned to dive, the rubber skirts were unforgiving enough that you really did have to take the time to try on every mask in the store to search for the one with the correct fit. I was unfortunate enough to find a mask back then that fit perfect! Unfortunately the ONLY one that fit was also the most expensive mask in the store.
In the end it really comes down to your personal preference. If you find a mask that you like the looks of, chances are pretty good if that particular one doesn't fit the way you like, another manufacturer has one very similar to it that will.
