I think it depends on what weight baby is expected to be and the nappies you will be using. Small nappies were far far too big for M and small longies were right up to her armpits and just looked ridiculous! It wasn't long before she fit in them though so you'd definitely get more wear out of them.
My small have 18" hips and 16" rise. To be honest in 5yrs i've only done a handful of pairs smaller than that (mostly for Louise ) so i would go for small unless you are expecting a tiny baby. At least if they are too big they can be grown into, but too small and there is nothing you can do