If weight is your primary metric for deciding between the two sizes, I would get the Check-In M.

I own both 67L and 85L model RIMOWA Topas trollies (now the Original Check-In M and L, respectively).

When I use the Check-In M, I typically cube-out before I weigh-out on the trolley (run out of space before hitting the weight limit).

When I use the Check-In L, I have to be more cautious, as I have routinely gone over the typical weight authorizations for checked baggage.

For any extra space I have in my Check-In L, once I reach the weight limit, I use the air pillow packaging (AirSaver, ULine, and MiniAir) that comes in many Amazon packages to ensure the contents inside my bag move around as little as possible.

I use my Check-In L more often, than the "M", since I would rather have the extra space available when I travel and pay the overage fee than not being able to fit anything more at all.