I had dr brown bottles with my dd and as much as I tried I felt like I could never get them clean enough. I would hand wash then dish washer over night to sanitize (because I didn't have enough to exclusively dish washer clean) I felt like I could never get then clean enough. One time I actually had went away for a few days not taking all my bottles and when I came back one of the vent system pieces had grown mold on it! I was completely grossed out and ended up going out and biting different bottles that same day.
I would suggest for a premie, not going with one with the larger nipples, like avent, My ds who was not a premie, had so much trouble getting his mouth around those. I had to go out and buy new bottles for him since the avent ones I had didn't work for him. His problem was he couldn't get his mouth on them properly and would end up pulling in air from the sides of his mouth. Basically making the venting system useless. I actually really like the playtex ventair bottles and the medela bottles for him.