I think you should look at the Flip system.
I used Green Kids for DD and tried about 4-5 before settling on them but I am using Flip for my son (9weeks) as it is trim, you don't have as much washing as you don't need to wash the covers everytime but mostly you can get disposable inserts too. They are also pretty cheap in comparison to all in ones and fit my son from 5 weeks and my 2 1/2 yr DD
Good luck, there are sooo many to choose from