I cannot answer in terms of the difference in the two companies since I have only used Mason ... but I do know another colored clay artist who uses US Pigments for specific colors that Mason does not have and he seems happy with the results.
As to pricing ... it is weird. Two different clay suppliers will have the same Mason stain for sale at wildly different pricing.
I have NO idea why, so it always pays to shop around.
As to why I have stayed with Mason ... have never yet had any problem caused by them.
I have however suffered self inflicted problems over the years and have called them for help and have been totally impressed.
This is a company that sells color by the ton ... yet their lab people treated my five pound issue with total respect and helpfulness.
Got a customer for life that way.
Completely understand - I've never really had any issues either so it's why we've always used their product. Talked with Mason's techs about a problem we did have once and like said they were very helpful and respectful for our "little problem".
They're the same products. US Pigment sells Mason. So buy the cheaper one.
Great to know! Figured this day and age there wouldn't likely be many manufacturers globally for this type of product, it all likely comes from the same place. Going to give them a shot and order a bunch of different stains.