Simple is good - don't be seduced by masks with drain valves, built-in snorkels, or the like; these are additions that are designed solely for the purpose of making you pay more.
Once you have a selection of masks that fit you, start comparing the field of vision they give you - some masks have extra windows, some have extra-large lenses. The more you can see, the better. Don't look just for width - being able to look down makes it easier to see your gauges and D-rings.
Once you have a small selection of masks that fits and give a good field of vision, choose the one with the lowest volume - low volume means easy to clear, and also means the mask will be less buoyant - some masks can make your nose ache as they fight to slide up your face!
Open-heel fins have a strap at the heel, and they come in a wide range of designs. Fin straps can wear out, and so can the clips - some people replace their fin strap arrangements with bungee or steel springs, which are much more reliable.
Most recently, the big gimmick has been split fins. The debate is still ongoing as to whether or not split fins really are a huge leap ahead or just a gimmick. The general consensus seems to be that some split fins are worthless, but some are at least as good as conventional fins. Working out which is which is no easy task. If you're determined to get splits, then you could do worse than check out Divernet - John Bantin is notoriously anti-split, so if he gives a split fin a thumbs-up, it's a fairly safe bet.
If you want to stick to a tried and tested fin design, then black rubber jetfins are about as safe as it gets - used by some of the most advanced divers in the world, and they've been around for decades. Scubapro (the original makers) sell them at a high price, but clones are available a lot cheaper from people like Divex and Sub-Aqua Products.
If your tastes run to something a little lighter or more colourful, then the other 'tried and tested' fin to look at is the Mares Plana Avanti Quattro.
Lastly, most fins are designed for one kick - the straight-legged scissor or flutter kick. Whilst this is the most common and efficient kick, it is in many ways not the best. If you use, or intend to use, other fin strokes, you'll have to do a bit more homework about what fins to buy - most fins are not suitable for more than one type of stroke.
No matter WHAT anybody may have told you, you do NOT want a snorkel attached to your mask at any time other than when it's being used. A snorkel attached to the mask is a snag hazard that can cause flooding or even loss of your mask, and is widely considered a sure sign of a novice diver.
If you're not convinced, in a poll on an Internet forum, results were:
- Snorkels are pointless, I won't dive with someone who takes one - 3 votes
- Snorkels are pointless, but I really don't care about it much - 17 votes
- Snorkels can be useful sometimes, but I don't take one - 29 votes
- Snorkels can be useful sometimes, so I take one stowed away - 13 votes
- Snorkels can be useful sometimes, I have one readily accessible - 17 votes
- Snorkels are vital dive gear, I won't dive with someone who doesn't have one - 2
So, over eighty votes. Two thirds of divers who voted do not even take a snorkel. And of the one third who did vote for taking one, some posted the modifier that they only took them when a long surface swim was expected.
Examples of folding snorkels you could consider include Scubapro, Sherwood, and Technisub
I use: Generic prescription mask plus Oceanic Shadow backup mask (folds flat for easy pocket storage), Sherwood Avid folding snorkel, Divex Jetfins with Manta steel spring straps