Dear M.,
I switched from Avent to Born Free because of my concern over bisphenol-A.
Avent has no extra parts other than the bottle, collar,cap and nipple. I kept the Avent nipples because I had read on other reviews that people preferred them with the Born Free bottles. The Born Free bottles come with Bottle, nipple, 2 inserts for helping with colic and gas and the collar and cap. I took out the two parts for colic and gas since my baby was past that point. This poses a problem. the nipples will collapse in on themselves and leak milk if you don't use the extra parts. I switched out the nipples for the AVent nipples and had no problem with collapsing but the nipples do not fit all the way up to the cap, so water, formula, breastmilk will leak out of the bottle and into the cap. You can keep the inserts in the bottle and use the Born Free nipples but then you have 2 extra parts to clean but if you have extra nipples from your previous bottles, then try those in the Born Free bottles. Otherwise, I have no trouble with the Born Free bottles.
I do agree with others that glass is also the way to go but at the time I ordered my plastic bottles, the glass weren't available and they weren't carrying Born Free at Babies R Us
M.