As for the hood thickness, the colder the water, the thicker your hood should be. If you are into deep dives or fear that cold water may be an issue, it would be wise to purchase a warmer 5mm to 7mm hood of a more thermal-friendly material (you can find an option with a fleece lining, for example). If you are a tropical diver, on the other hand, and worry more about the protection from the sun, Lycra hoods are just what you need.
There is also a distinction between the wetsuit diving hoods and hoods worn with drysuits. Wetsuit hoods cover the head and the neck areas, as well as include the bib, that can be tucked into the wetsuit at the neck opening. This bib reduces the transfer of water and keeps the diver warmer. A drysuit hood also covers the head and neck but is designed to cover only the neck seal of the drysuit.
In case you want additional thermal protection, consider easy-to-wear hooded vests that are also available here, at Dip ‘N Dive.