Fins are the primary source of motion while Snorkeling and scuba diving. Comfort, flexibility and power are important physical aspects of the product to consider. Other variables that should come into play are price, colour, quality, material and style.
There are two styles of fins, open foot and closed foot. Closed foot fins are usually a bit lighter and smaller than open ones. They also can be used for scuba diving although most divers use open foot fins, especially for cold water diving because booties are required. Generally speaking, closed foot fins are more flexible than open foot fins, making finning easier and faster. Bigger divers with large feet, especially men, use open foot fins with straps and booties. It is easier to put on open foot fins with booties than closed foot fins.
The colour of your fins might be important in low visibility waters and as a point of recognition to other divers.
Enjoy your dive or snorkel!