Sleepers are still a great clothing option for young babies but there are some pros and cons to snaps vs. zippers: (Carter’s line calls this type of clothing “Sleep & Play”)
They are especially great for cooler weather or when you want baby to wear one piece of clothing (instead of a shirt, pants, and socks). They are also great because they do not go over baby’s head; they fasten in the front.
I really like to use the zip-up versions when going to the pediatrician’s office. The nurse asks you to undress baby down to a diaper when they weigh him/her. So this makes undressing there really easy and simple. Just unzip!
Sleepers with zippers are faster and easier to put on, but the whole thing will be unzipped for those quick diaper changes (with only baby’s arms still dressed).
Sleepers with snaps are like rompers: you only unsnap the bottom three snaps for diaper changes.
They have footies and are always long-sleeved (has anyone seen these in a short-sleeved version?) which is great for cold winters but once baby’s feet fill up the footies, you should go to the next size.
(Update: Carter’s makes a footless version of the Sleep & Play, available in the Spring/Summer; also search for Carter’s jumpsuits)
Again, my recommendation is to use cotton knit sleepers, not fleece. If you have really cold winters, you can try the other fabrics, but I’d rather just layer with a sleep sack or flannel blanket. Don’t overdress your baby for sleep.
So you have to ask yourself which type of sleeper you’d be willing to deal with at 4 AM.