A purge valve is worth the extra $10-15 you might pay for on a snorkel, and a purge valve isn't usually much bulkier than a regular J-tube. If you're snorkeling too you might want a splash guard, though with the purge it's not absolutely necessary. Also, if you are getting one with a splash guard try to get one that is removable. That way when you dive you can take it off and it won't hook in your hoses or cause other problems. Any other gear is just useless crap that gets in the way.
The collapsables are handy for scuba, but most I've tried aren't worth bothering with. I'd say if you spend more than $40 on a snorkel you're probably throwing away money for nothing, especially since as far as scuba goes you'll be using it for maybe 5 minutes per dive. And for snorkeling you really shouldn't be out when waves are breaking on your head, so a basic snorkel is adequate.
I don't know if you are a freediver, but if you are you might also consider that bigger snorkels are going to be more difficult to breathe through.
This doesn't need to be complicated; my advice is find a snorkel, try it, buy it, and forget about it.
I've been living in Maui for the last two years and I've never heard about the snorkel requirement, and I've been on Ed Robinson, Mike Severn, Maui Dive, and one or two other boats.