when i was ep'ing for avery (30 weeker) we got caught up in the hype of the Avent craze but came to find the nipples weren't maleable at all and she didn't have to work very hard (compared to nursing) to get anything, so it wasn't helping her transition between the two (she eventually learned to nurse 3 months after birth and became an exclusive nurser). She could literally just open her mouth and put it on those Avent nipples and the milk would fall out of the bottle.
We switched about a month in to Playtex Natural Latch nipples and never looked back. To us they were the closest to MY nipples - which is exactly what I was looking for. Squishy and the right shape so she had to work it like a real nipple to get anything out. I really think it helped her "click" into the rhythm of nursing better too (whoever thought I could thank Playtex for helping my baby become a boobtick until 3 1/2 years old??
Anyway, long story short, my $0.02 is to skip the Avent (SO expensive anyway), start with Playtex Natural Latch, be it with regular or drop in bottles (we used drop ins... less air), to stay as close to breastfeeding as possible. If you find that the Natural Latch aren't comparable to YOUR anatomy, what your baby is used to, then seriously go down the aisle at Babies R Us or whathaveyou and examine the different types that are out there. Find something comparable to YOU in terms of flexibility, shape, and size, and your baby will have a much easier time going back and forth. JMO.