I just bought a new snorkel after 20 years of use on my prior purchase. I chose the TUSA SP-170 first for comfort of the mouthpiece and bore width of the tube. Next, I evaluated how water drains from the tube. The purge valve under the mouthpiece is covered, so stray sand or kelp will not block it open and let water in, a problem I’d had with older snorkels.
At the top of the snorkel tube, TUSA’s Hyperdry System creates a separate pathway for water to eject, making for quicker clearing of the airway for my next breath. Other brands do have similar configurations and differ only slightly from the TUSA design. What hooked me on the TUSA is its Comfort Swivel, which allows me to change the angle of the snorkel without messing with the mask strap. It also has two parts that can disconnect as a quick-release to get the snorkel off the mask quickly. Using the old snorkel keeper strap was always a hassle for me.
Snorkels are a very personal choice, and the number of features surprise people who have never purchased one at a dive shop. Some stores won’t allow you to put the mouthpiece in your mouth. If you can’t judge the size/fit, try to see if they rent the model you are interested in. Usually an experienced salesman can judge the size well and you can go by his suggestion.
I’m very happy with this choice, and have found it to meet all of my needs either in surf, open ocean or pool conditions.