Comfort and ease of use. Whatever you think will be easiest for you to a) carry, and b) get stuff out of with one hand.
Life with an infant is all about learning to do everything one handed. You will want to be able to grab your quick-need necessities without having to put down baby (or even the bag in an ideal world). So outside pockets/pouches are an absolute must. You'll want to able to grab wipes/receiving blanket (for spit up/barf/other gross things that babies insist on doing at the worst times), pacifier ("OMG please don't cry here!"), and bottle ("OMG you're still crying... Help!") without missing a beat. Everything else you'll be setting baby and bag down anyway, so the other stuff can all be zipped inside.
The backpack may work, but the shoulder bag is easier if you're carrying baby. (and you'll be holding him/her much more than you may think - I swore I'd never be one of those moms that carried their baby whenever baby wanted, but I was so in love with her I just couldn't put her down lol)
Aaaaahhhh... It's been so long since I've had a baby baby! [emoji7] [emoji7] [emoji7]