I think it totally depends on your LO... from my experience I didn't notice much of a rise difference between the OS and the medium, and both my 11 month old and 25 month old can wear the mediums just fine, with room to grow. Personally I prefer the sized ones, but if you have a skinny and tall baby I would say the OS might be better since it is more adjustable.
our baby is at about 16 pounds right now (5 months). the FB OS is noticeably narrower at the crotch than the fb M. we use fb (M) as a night time dipe and the fb os very rarely ... just not as thrilled with the fit and smaller room for the soaker.<br><br>
Holding the 2 dipes up together, the OS back and fronts are the same width. My fb M (from last year - no hip snap) has the same number of snaps in the lower row, but 1 less snap on each side in the upper row... it will adjust to the same max size for lower row, but about 1" smaller in the upper.<br><br>
FB M fit baby starting at 10-12 pounds, we used prefolds before that. I would go with FB M unless you expect the baby to grow very slowly to 12 pounds. (preemie for example)