I agree with Damsel. Fit is the absolute most important thing when considering a new mask. If you paid 1000 dollars for a mask and it does not fit properly then it is just a 1000 dollar paperweight.
To get a proper fit:
-Try the first mask on. DO NOT worry about the strap. In fact leave it off.
-Look in a mirror. Make sure the inner seal is making contact with skin all the way around with out inhaling. The inner seal should not be running over your eyes or through eyebrows. If it is it is not contacting skin, making it more likely to leak.
-Make sure you nose fits the nose pocket well. Same with the frame between the eyes, it should not contact the skin. The lightest contact on the surface will become a major problem in the water.
-Compare the 2 masks at a time. Pick 2, try them on. Hold on the to best fitting one, put the other back on the shelf, grab another one. Compare the one you kept with the one you picked up. Eliminate one, grab a new one. Repeat until you have the best fitting mask. Comparing 2 at a time is easier than more at the same time.
-If the one you end up with is an uncommon, or hard to find mask and your traveling buy 2. That way you do not have to spend your vacation dive time trying to locate a replacement for a lost, damaged, or stolen mask.