What are the different kinds of bags?

There are two different kinds of bags: hide/skin and synthetic.

1. Hide bags are made of cowhide and are commonly tanned using a process used for elk - hence they are commonly also called elk-hide bags. These bags have very little porosity and will retain a moisture longer than other types. While not a good choice for frequently playing in cool, wet environments, these serve well in most of  the continental US. Usable lifetimes of 7-10 years are common.

Sheepskin bags are technically also a hide bag, but they behave different in that they have high porosity. These are best suited for moist blowers/and or climates (e.g. Scotland) where you want to loose a lot of moisture. They may need a lot of seasoning if played in a drier climate - e.g. once every 2-3 weeks may not be unusual in the heat of summer in the US. Because of the high porosity, sheepskin bags are less rugged than the other choices and often only last one or two years.

2. Synthetic bags are made of a semi-permeable material (e.g., Gore-Tex) to release moisture. Many top bands and top solo players use these bags because they control moisture very reproducibly.

The "lack of substance" and "floppiness" of these bags has been a criticism. These bags are more somewhat more unforgiving of variability in blowing pressure than leather, but some instructors feel that these bags help teach proper blowing technique.

Synthetic bags should NEVER be seasoned.  To do so ruins the porosity of the Gore-Tex material.

Synthetic bags have had some problems with mechanical failures in the drone mount grommets.  That problem seems to have been solved, but I've seen two failures in which the bag, just below the blowstick becomes quite permeable, as if saliva has digested some portion of the bag.  One manufacturer now issues a warning to leave the bag open with the blowstick removed when not in use - to dry out the bag and prevent enzymatic activity.  All these bags are now available with "zippers" to allow access to the bag to retrieve reeds and to allow the bag to dry out.  Other minor issues are that the zippers can fail.

There are three major types:

Simple cloth bag:  The Canmore Gore-Tex bag was the original synthetic.  Other makers including Ross and Bannantyne.  I believe that a tube-type water trap is essential in these bags.  It provides additional structure which makes a huge difference in making clean cut-offs while simultaneously also solving the enzyme problem.   I've been told that these bags can be patched with inner tube tire patch kits and have used silicone sealant on one or two as a temporary fix.  Another major issue I've seen is that some bags are installed (and in one case shipped) without the heavy O-rings to retain the stocks in the grommets and stocks can "blow out" of the bag.  Replacement O-rings needn't be as heavy as the originals.  Heavy rubber gasket seals for commercial farm tractor sprayer attachments can be used off the shelf.   The Gore-Tex bag can be laundered (and dried) if desired. (I rinsed mine with a few drops of liquid dishwashing soap and warm water once each year to remove the residues of saliva.) With proper care, the lifetime of these bags is unknown, but guaranteed for two years. I played  the same one (with a water trap) for about seven years.

Hybrid Hide/synthetic bag:  These bags were originated by Shepherd and are now also made by Bannatyne.  These bags are an attempt to provide structure and feel to the bag while using the properties of Gore-Tex.  Basically it's Gore-Tex on the inside and leather on the outside.  There's a zipper for access/drying and a simple spit trap. 

Desiccant Bag:  A newer variation of the synthetic bag is the Ross Canister bag which incorporates a desiccant system.  More recently, manufacturers have begun to produce separate aftermarket desiccant systems for other bags.  The Ross is a semi-permeable synthetic material bag with a built-in humidity control system based on "kitty litter". The player fills the bag, and the air passes through a centralized four section distribution box filled with "kitty litter", one section for each drone and the chanter, to dry the air before passing into separate tubes for each drone and chanter stock. Stocks are held in the grommets with hose clamps. This bag has received very positive reviews, even from open class players that I've spoken with about it. I do know of a piper who gave up the bag because the distribution box resulted in bruises on the arm and ribs. I don't have that problem with the bag, but I have found that it can take a very long time to equilibrate the humidity in a wooden reed while playing this arrangement. For this reason, I've chosen to remove the kitty litter from the section leading to my chanter reed while leaving the kitty litter in place with synthetic drone reeds. I've confirmed that this approach is also used in world class bands. The system really does seem to reduce moisture buildup in the drone bores which is a real problem in cold weather (e.g. a funeral). A significant negative is that these bags don't fit into a "normal" pipe case.

If you do decide to try the Ross Canister bag, spend the extra money for the zipper version instead of the clamp as the latter is a pain to use.   (Sizes are not "marked" on the Ross bag.  When shipped, a small yellow square of tape inside the blowstick grommet indicates that the bag is a medium size.  A grey-silver square indicates that it's an extended small.  There is also considerable discussion/belief that the proper size for most people is the "extended small" bag - not medium. Both bags are the same length.  Other sizes are similarly marked with tape: red denotes "large" and green denotes the "Livingston small" bag with the drones moved forward by about an inch.  )

Recomendations:

As you might have guessed from that last comment, I don't play that bag.   I play a Ross Bag.  The moisture control system is the best out there.  I like the feel/cut of the bag and the fact that the zipper is on the bottom.  However, it is a hassle to maintain the desiccant system.  It is more expensive, harder to install and harder to store.  Hence,  I just can't recommend it to everyone.