There are many absolutely wonderful and suitable bags available today, all of which are measured against the sheepskin pipe bag for comfort, control, moisture management, and contribution to the sound your bagpipe makes. This is very similar to synthetic drone reeds and their comparison to cane reeds. We’re not quite there yet with chanter reeds however there is much work being done in this regard, so I expect a bombshell announcement one day. Let’s get back on topic.
Your choice of bag might be determined by your Pipe Major, which I wholeheartedly understand and support. Much of this has to do with consistency of sound, predictability of behavior, and managing the weather on any given day. Just for instance, when I arrived in Tucson, there were several different bags being used in the band. After a bit or research and talking with trusted friends, we made the decision to switch everyone over to a goatskin bag. We also tied the stocks directly to the bag rather than opt for rubber grommets. This turned out to be the right decision for us and certainly for me. It made setting the band up in 100° F and 10% relative humidity much easier. Anyway, if you’re in a band, go with what the PM decides.
If you’re piping on your own (no band) the choice is yours based on self-determined criteria. There are so many good choices out there, it’s hard to go wrong.
I will reveal one prejudice that I carry. Sorry, but the thin, flimsy, Gor-Tex bags may be inexpensive and easy to tuck into the pipe box however they make starts and stops very challenging and do nothing to bring out the best in your drone sound. I may feel differently someday, however I’ve seen too many inexperienced pipers struggle to be a fan of these bags.
