MASKS

Never rely on borrowing or renting a mask. It's not worth it. An ill-fitting mask can contribute more to a miserable dive than almost any other item. When you find a good mask with a wonderful fit, take care of it and hang on to it for as long as it hangs together. Hence the need to be particular about its quality in the first place.
I have often told people that it's faces that leak, not masks. In reality it is the interface, the seal between your good looks and the mask skirt.
It is difficult to know when you try it on in the shop if the seal is going to hold its shape and give long service, but at least make sure that it gives a perfect seal from new.
Once, mask skirts were made of rubber and you could tell because it was black. Then you could tell if it was silicone because it was transparent. Then it became more difficult.
Nowadays, silicone can be many different colours and the gap between silicone and rubber is filled with numerous other compounds offering different characteristics. A mask skirt in a silicone-type material really is a better buy as it will last far longer than a less noble material. It will fit better and give less chance of skin irritation.
The better brands are less likely to suffer from irritating fogging when new, because they take a little more trouble to clean off the de-moulding products before packing them. Hence they will be a little more expensive. Money well spent.
Make sure that the strap adjustment is quick and easy and needs only one hand. You will probably be holding on to the anchor line/ladder/your camera with the other.
Give a thought to the overall weight of the mask. A lighter mask is more likely to stay in place with the minimum of strap tension.
Masks come with anything from one to six panes of glass, but this is not necessarily a case of the more the merrier. Probably the closest fit is achieved with a two-lens mask, giving a low internal volume, easy clearing and light weight.
Additional pairs of lenses give additional vision (of sorts) at the sides and below the normal range. Make sure this is fulfiling a personal need as it will involve extra cost and weight; the Mares Eva 6-lens mask tops out its range at £80.
The plastic frame is typically assembled using its own moulded-in clips, so check that the matching frame components fit tightly together. A little gentle flexing of the frame sometimes exposes potential leak points.
I was surprised to see some top-of-the-range masks produce major leaks for some of my dive buddies last season. Not a dry eye in the house.