I never used either of those. But this week I bought fast flow latex nipples for my drop-in playtex bottle. I'm actually liking it a lot better then the slow nipple it came with years ago.
However it still takes a while in comparison with drinking like normal. Though I suppose that can make you drink less, just like eating shell-on pistacho's can make you eat less and feel fuller.
I'm not sure if an Avent one would fit. I used a Playtex nipple on a Gerber bottle before, and it worked...sort of. The Playtex nipple collapsed far easier than the original Gerber nipple.
I don't think it would, I have an Avent bottle and the base of the nipple is fairly wide, and looks as though it would be too big to fit in that bottle.
The shipping+handling=Crazy 18 dollar's usa x.x
It has been my experience that bottle makers tend to make their bottles & nipples just a tiny bit different from their competition. Different brands might fit...but the chances are it won't be a perfect fit. So...yes...you'd be better off getting a Gerber nipple for a Gerber bottle.