After measuring my wrist size according to apple's website it suggests either size watch would be ok so that doesn't really answer my query .

Personally I would prefer to test fit to give me an idea on size comparison hard to do if you might be purchasing used or no available shops to try on.

Prints are for the last 2 generations.

apple watch 6  - 44mm vs 40mm 

apple watch 7  -45mm vs 41mm 

note : ignore the bubbling on the top surface I think this is down to a 0.2 speed layers and 5% infill