First decision to make is - will you use it for snorkelling, or for diving? If its main purpose is as a back-up for surface swimming before and after diving, stowability is a major factor. If you have been taught to wear your snorkel attached to your mask-strap at all times, you do not want it to interfere with your mouthpiece at other times, so a "fall-away" flexible lower end could be an advantage.
If you prefer to carry it stowed away, you will probably prefer a smoother profile with the minimum of bits to get caught in straps.
For general snorkelling these problems do not exist, as you are using it all the time. The major considerations are then to do with comfort. You may be snorkelling for hours, so ensure that the mouthpiece fits well and has no sharp edges.
A snorkel with a drain valve at its lower extremity will stay significantly drier, as it does not store water in the air passage but expels any stray drops with every exhalation.
This lower valve is far more important than any upper-end valve designed to restrict the entry of water. It is difficult to prevent water entering without also interfering with the passage of air. And any water that does enter is usually handled by the lower valve.
When US Divers first introduced its Impulse snorkel, with its mid-tube annular valve and lower drain valve, it raised many eyebrows, but it went on to be a best-seller due to its dry breathing performance.
It helps if the part of the snorkel protruding from the water is in a highly visible colour, even if this plays havoc with your otherwise colour-coordinated kit. It could be the only way you can be spotted.
Most manufacturers offer a model suitable for children or those with a smaller mouth. They usually fall at the lower end of the price scale, which is good, but check that they have not skimped on assembly technology by simply jamming the mouthpiece on to the end of the tube.