I'm taking advantage of the Diapers.com sale for grandma's house and trying to figure out which size to get. She should be in a size 3 (16-28 lbs), but she can wear that now and we won't be visiting for a few months at least. At max she'll probably be 24 lbs by then. Should I just order the next size up? What would you do?

Do you find that having a diaper that fits *just right* is better for blow-outs? Is it better to be in the smaller or bigger end of the diaper size to prevent leaks?

Size 3 (16 - 28 lbs)
Size 4 (22 - 37 lbs)