I did a mixture of both, mainly thinking that 0-3 wouldn't be that much too big for her but would last her longer. I ended up needing the MiL to go out and get more newborn clothes while I was still in hospital as she kept going through at least 3 vests and babygrows a day and would always look like a pretzel when she wriggled her legs out of the 0-3 babygrows.
As someone said, you could always get some newborn stuff and keep it in its packaging with the receipt and see if it's needed when the time comes.