I'd recommend getting a small pack of both sizes you are curios about. My daughter was in 4s when I put her in the 2t-3t easy ups and they are still a little big. My daughter has always had a small waistline though. Love the easy ups over pull ups. Much better at getting the moisture off her skin.
Pretty good! I put them on my 3 year old just for bed and then she uses the potty all day. I have only had a problem with them when she drinks a lot of water right before bed and sleeps in late. Other then that they work very well!