It's a hard call! Smalls didn't fit Ruby till she was about 6 weeks old and well over 4 kilos. They still fit well now though we are having rise issues so she's becoming too long for them.

She's under 7 kilos now and mediums gape around the legs

FWIW C was a big bubba and he fitted itti smalls till he was about 5 months so he wasn't in them long so I sold them off and got the next size.

It all comes down to how bub is built really. Doing what you said is a good idea because you get to try both sizes.