Funny, not sure what made me click on this thread again but I was just browsing through to see if anything had been missed and stumbled upon this. Anyway, not sure if you got an answer since you posted but thought I'd do my best and bump it up at the same time.
Basically it's just a size difference. One is for newborns weighing 6-12lbs while the other is for slightly bigger babies and goes from 8-20lbs. The smalls would be fine for the vast majority of newborns, they'd fit fine and wouldn't leak. But the x-small offers a slightly more customized fit and will be trimmer than the smalls. Tradeoff is that they'll be outgrown much faster than the smalls. The x-smalls are either for those who are looking for the most customized and trimmest fit possible for their newborn or perhaps those who have or are expecting a preemie or particularly small baby.