Well, here are my two-cents, if they're even worth that much :shame: I'm one that says a Birkin will be a pain with a new baby - even a 30cm has to be hand/arm-carried and could get heavy and bulky while you're also toting around an infant carseat/carrier and diaper bag all the time. I'd just be afraid of it getting bumped around. Save the Birkin as your second H bag, when your kids are older and more self-sufficient.
For now, I think the Kelly is a great choice and this is the PERFECT first H bag! Just use the strap when you're arms are full of baby and baby things and you're set!
If you want an everyday Kelly, I don't recommend the black box rigide for you at this time. Kellybag has a gorgeous 28cm and I can see why she doesn't like to carry it around with her kiddos in tow. It's a special evening bag, or when she's by herself (if that ever happens, right KB? :
)
But, a 32cm souple style in Togo, clemence or Chevre leather with either gold or palladium HW (don't know what you prefer) is an entirely different and much more user friendly bag. I'd definitely get the 32 because when baby girl is a little bigger and doesn't need a diaper bag, you can always throw in a snack bag, wipes and a diaper and be ready to go. This is coming from a mom with two small kids who also loves to carry around nice H bags everyday!
As far as color, Vert Anis is gorgeous and go for it if you think you'll use it enough with your wardrobe. Otherwise, check out Rouge Hermes, Gold, Potiron, or any of the chocolate browns.
Whew! That was a book, but I'm so excited for you and this is going to be the best push-prize ever!