I've never heard of Estrin, but it's apparently a generic conjugated estrogen. Premarin is a brand name conjugated estrogen. I'm sure you already know that though.
The whole deal with generics is that they are equal substitutes to the brand name medication with the exact same formulation, since its patent has now expired. There seem to be a lot of people who are afraid to use generics. In reality it's the same thing, but it costs less. Think generic foods: costs less, tastes the same. Generics are perfectly fine, and would not be prescribed/would not exist if that were not so. I might be very wrong, but this is my take on the issue.
If Estrin is causing heartburn, then standard Premarin will probably not be any better. Also, antacids can interact with medications, and can alter the potency (I paraphrasing from a package of Tums). Be careful.