The trainer told me that one snug and the next size a little loose but not enough to affect him. She said as he matures he's going to shift some and if I buy the smaller sizer and don't use it this year that it may not fit next year and I will have wasted my money. It kind of made sense to me because money was tight and I never bought infant clothes under 6 months or shoes that just fit:') I think you will think his fits ok but SL will look like she's a knights horse:')
I wouldn't imagine either of them will change much in length chest to tail... but as long as you make it fit, it works!