That (your response) doesn't tell me anything... I suppose I could guess.
If I knew that the $40 battery was really a quality battery it would make the decision easier. But I have no assurance that it is. I see complaints about Batteries Plus too. And looking on the ATT site I am not sure about those either - a lot were from second sources. So "you get what you pay for" may not apply - paying more doesn't necessarily mean you get more.
I would just get a new phone for cheap but I like this one, have a case for it, etc. If I could find a new model that took the same battery I could get it and just use the battery in the old phone. Funny that you can get a phone with battery for the price of a battery (or less) but I know how markup works on replacement parts.