Solo and rescue are both about anticipation, prevention, and recovery from incidents. Rescue is with others. Solo is with no one else there to help. So, to me, Solo covers more than Rescue, for the diver themself. Thus Solo trumps Padi MSD. Not that you should not take Rescue, but for the diver themself Solo covers more.
(ETA: Well, except for the bandaging yourself up part of EFR that is a prerequisite of rescue.)
Cavern has a few problem-solving drills and emphasis on good trim and control, applicable anywhere. Which MSD does not add.
Cavern Diver | PADI:
"You'll complete four scuba dives with the first focused on line handling, reel use and emergency procedures outside a cavern; and the next three dives put your new skills to use inside a cavern. You'll also learn about:
- Other specialized equipment for cavern diving, such as lights and redundant breathing systems.
- Avoiding disorientation and anti-silting techniques.
- Dive planning, including depth and distance limits and air management for cavern diving.
"
Also a good argument for trumping Padi MSD aside from the 50 dives requirement.