Hi, it's up to you. If you want the suit to fit, go for S size. If you want to wear longer then take M size. We understand that babies growing fast so we have this "Trade In Program", for more information please log on to http://cheekaaboo.com/store/index.php/trade-in-program/
This particular design you chose has only 2 sizes, S: 6-18months and M: 18-30months. Due to we use velcro, it will be more adjustable for your kid. May i know how old is your baby?