This thread looks familiar- we've done this one before surely? I'd say, don't ask your child. They have no idea how they'll feel when the baby's been born, in a year's time, or in 10 years' time or when they're grown up- you are the ones that have the experience to think about all this. Besides, children change their minds. If she's been consulted once, she may feel she has a say in these matters- and be tremendously disappointed when you refuse to send the baby back! Also, a pregnancy of 9 months is a horribly long time for a small child to wait, not even mentioning that you may not fall pregnant straight away. Again, she may feel disappointed if you've promised her a sibling and it then doesn't happen, or it doesn't happen until she's changed her mind. So I'd leave her with a fait accomplit.
There are lots of arguments for single children and for having siblings, so I wouldn't feel bad for her whichever you go for. I'm sure she'll have a great life either way.
About the best age gap, I would think first about what is best for your husband and you. Siblings can enjoy each other with all sorts of age gaps. In particular, I would consider what the chances are about waiting (your age etc), is there a risk if you put it off that you may not conceive? Are there any career considerations etc?
My kids are 3.5 years between them, simply because it took me longer to become medically fit and then conceive the second time. It's not a bad age gap, they enjoy each other a lot, but I see other types of age gaps around me that also work well. I am very close to my younger brother (16 months between us).