|I haven't used the AIOs, but if you do a search on the forum, you can probably find comments that they run a bit small. Or at least that's what I seem to recall reading.
How long S Sandys can be used can really vary by baby. My DS#1 was out of them by 17.5 or so, while DS#2 was able to wear them to maybe 22 or 23 lbs. If she's short, they'd probably be fine for now, but for the money I'd get the L, because they can be used at even lower weights and will get you to PT. If you're concerned about the bulk, maybe bamboo? You could always get one of each (maybe the trial package?).