I don’t know where you got the idea that Andover has more laid back teachers than Groton.
The only reason to choose Andover over Groton is because you really crave a huge school v a small one. People who know, know Groton, it will not hold you back college or job wise. Groton is 100% as academically rigorous as Andover, if not more so. Unless you are playing a varsity sport you will have plenty of free time at either school. If you play a varsity sport, you will have almost no free time at any school in season.
Dress code - Groton’s is written more strictly but in reality it is not intrusive in any way. I have never heard a student complain about it.
Groton is incredibly selective and being admitted is very impressive, more so than being admitted to Andover. You should feel proud of yourself. Groton often has spots for kids consider to out of the mainstream for other schools, their inclusiveness is a way of life much more so than schools of equal prestige.