Im not sure about one vs the other, But I will say that we had some old air plane gas tanks that we were going to fill with foam to make toons out of them. An engineer friend of mine said that there is no added bouyancy benefit by filling them with foam as long as there is no leaks. It would have been just wasted money. Now adding foam would prevent less water from being able to get into it should it develope a leak. So if your bottles aint leaking would be cheaper to not fill them with foam. My intention was not to hijack your thread. I apologize if it appears that way. Just trying to save you some money if you were planning on filling alot of bottle with foam. My grandpa always told me, "It aint what ya make boy.....its what ya keep." good fishing to ya my friend!