Completely personal preference. The advantage of the SIO is that the shell can be reused with new inserts, provided that the nappy is just wet, rather than being just used once then washed. Pulling apart for washing also means that the system dries quicker, so the outer can be used with new inserts before the original inserts are dry. The AIO is slower drying, but it is a true AIO; no need to faff about reassembling the nappy, just take it off the line and use it. I guess it comes down to your washing routine.