I have bought both sized and BTP nappies, I think the sized are a better fit generally, especially if you have a small, odd shaped or especially large baby (lol!) hen you can buy a more accurate size. I know that my BTP nappies wouldn't have fit Freya when she was very young like the Little Lambs did. I also used Motherease Sandy's, Fuzzi Bunz and Bambeasy nappies when she was very small and they all fitted really well. Some of them were Extra Small sizes.
I'm switching over to mostly BTP now, I hope they'll fit for the rest of her nappying time!