How old is your baby? Avent bottle nipples go like this: Newborn, 1+m, 3+m, etc. Are you using the 1+m or the 3+m? I have a 3 month old and I started using the 3 month one, and he seemed to be getting too much milk. I didn't realize that they made so many nipple sizes.
I personally like the Nuk bottles better (their nipples are 0-6m, 6-12m, 12-18 etc. so we will be buying fewer nipples because by the time he's 18 months, we want him off the bottle). He also prefers the Nuk over the Avent (he doesn't choke and he doesn't get frustrated with it).