So far so good with the Sweetwater epoxy for me. Its been holding good for over a year now. Real easy to mix and it has a nice working time. It was the first time I had worked with 2 part epoxy and I had no problems.
I have no idea on durability or lifespan. Other solvent based epoxy paint like Sweetwater have lasted for at least two decades, however.
Ease of use is a tough one. The epoxy paint will likely be easier to apply but hard to clean up and have a much nastier smell. The Pond Armour is nearly odor free from what I understand but maybe a bit more difficult to apply since it sort of has to be squeegeed onto the surface.
I would also look into what the coverage at the recommended thickness. It might turn out that one is cheaper than the other once this is factored in.