Most important (for us, anyway) is t-shirt quality and softness. Like you said, we buy big and they grow into it and out of it but it's the quality that really matters. We buy mostly Hanna Andersson shirts for our boys. They're expensive but they still look good after two seasons of wear, which is remarkable for kids' clothing. Our younger boy wears many of the HA shirts and coats that his big brother once wore.