I would recommend these 250ml bottles if you need to pump a lot (I am pumping all my breast milk). I originally bought the 150ml bottles but I was having to change them over mid-pump (time consuming) and it also created more washing up. If you are looking at these bottles you already have a Medela pump, so there is no point reviewing the finer points of the whole system here, but I would just note that I don't like the lids much. You have to handle the two component parts to assemble them even though the instructions that come with the pump say to avoid touching the insides of bottles and caps after cleaning. The lids come apart so you can use the Medela teat. This is good in theory so that you don't have to transfer the milk to different bottles. However, I found the Medela teat too 'narrow'; my baby was taking whole teat into his mouth, whereas the Tommee Tippee and Avent teats are wider at the point where they fit to the bottle and more similar to a nipple and breast. To sum up, if you are pumping a lot, don't waste your time on the small bottles.