No other piece of equipment has a greater affect on your diving performance than your fins. Not only do they have to be very powerful to get you back up again when your suit has lost its buoyancy but they also need to be light and efficient so you don't have to burn too much oxygen driving them. As always fit is supremely important so try on different brands of footpockets as they will all have slightly different shapes. Never buy open-heel, strappy fins. They are less efficient and the buckles create a huge entanglement risk.
There are basically three types of blade; plastics, fibreglass and carbon-fibre. Plastics are the most basic and cheapest but simply do not have the snap or performance of composite blades. Two-part fins are a good choice to start with as it is possible to change the blades out later on.
When looking at composite blades, get ones that have a tapered blade as this creates the most efficient curve. Fibreglass fins tend to be a bit heavier than carbon and don't have the same 'snap'.
The stiffness of the blade plays a large role in the performance of the fin and therefore the diver. In general a softer fin is better than a harder one. It doesn't matter what depth you're diving or how strong you are the softer blade will give better performance. This does need to be matched to your body weight though as what is a nice soft blade for a 70kg diver will be a limp noodle if you're 100kg.
Other factors that influence fin performance are the angle of the toe. There's varying theories on what the optimal angle is but it is generally about 30 degrees.
Full length rails are also good as they prevent water slipping over the side of the fin and protect the edges.
A quick word on fitting composite blades to footpockets. Virtually any blade will fit any fin with enough persistence but it does often require an angle grinder to shape the end of the blade. Your best bet is to match up your old plastic blade (or the one the pockets were made for) against your new ones and cut them to be the same shape. When fitted correctly the kick of the blade will match the kick of the footpocket.
SHOP FINS HERE