I inherited a set of motherease os and some large sandy's when I was pregnant and I love them. They (the os) fit from birth until present (27 lbs.)and there's still room to go.
My mother bought me another dozen when ds was born (these were the unbleached kind, while the ones I inherited were bleached) and these were really too big until he was probably 12-13 lbs. So I don't know if they started making them bigger, the unbleached are bigger, or the dozens of washes the first set went through with their first owner shrunk them, but the unbleached ones are still bigger on ds at 27 lbs.
I just started really liking the Sandy's recently when ds started having huge soaking pees (they're definitely thicker and more absorbant, but take longer to dry) and they also fit better now that he's walking??? .. .though I'm really not sure what the size range for them is.
For a long time we used a os with two liners at night and they worked just fine. Now that his pees are so big we moved to pockets at night, and I'm going to try some other things too, but the meos or sandy's were just not cutting it.
I really like both the covers, the only reason I prefer the air-flow is that they're easier to get on a toddler trying to run away from a diaper change. I find that they both work equally well. When he was a medium, I only had the Rikkis. They're really thin and sturdy.
Except at night, I've never had any leaks with the whole system at all unless we put something on wrong, which is pretty hard to do.
All in all, I think they're great. I can see my os, which I got after my friends baby was exclusively in them for 3 years, and used exclusively for my ds, lasting for at least another baby, maybe 2.