jviolin, I do a lot of freediving. I prefer a snorkel that provides the least resistance to breathing and swimming.
For me, that has been large bore, short (just a bit higher than the top of my head when in the water), and curved to fit the side of the head.
No purges, keep-drys, drains, etc. I never have a problem without these, and all it takes is a sharp puff to clear them anyway. All these gadgets have done for me is restrict my breathing.
I'm paranoid, because it's very hard to find good high performance snorkels, and it would be hard to replace my large-bore Farallon custom mouthpiece snorkel. If I had to replace it, though, I would consider the
Omer Zoom Pro .
The ability to freedive well should be a skill for all divers. Generally, good freedivers are more fluid and efficent underwater as scuba divers, than those who haven't spent time learning freediving.
If you can, try before you buy.
All the best, James