What is worse: an open fracture or an ACL rupture?

Both. Both are pretty severe injuries. I guess it depends on what you consider bad. In general, an open fracture involves injuring the tissues around the bone that is broken, so it also depends on which bone and what severity. An acl rupture, while very bad, can be reconstructed close to the original. In the end, both are bad but hopefully, we can help and manage both situations.
Open fracture. An open fracture is worse. Higher risk of infection and other complications and/or longer recovery. Open fractures always require surgery.