Blanket size 40”x40” or 35”x42”?

I’m making simple granny blankets for my 4yo and 3yo in a way that can potentially be added to later. I can do 5x5 with 8” squares or 5x6 with 7” squares using the yarn I have. I think either will look nice, just can’t decide square or rectangle. What do you think?

(Please tell me if my math sounds wrong in any way! It is not my strongest subject haha.)