I need to organise a couple of thousand mounted 35mm slides into archive storage, arranged into groups. Currently they're scattered all over the place in projector magazines and slide holder boxes.
It depends. If you have sorted them for shows and often show them or need to be able to prepare a show in short time, then those slides should be stored in projecting trays. For the straight ones exist stackable plastic boxes. Out of which one can draw trays without rearranging the stack.
If used as achive I would tend to small boxes.
The transparent binder-sleeves are something inbetween.
This all concerns practicability. Another issue would be longevity.
I learned by bitter experience not to store slides in projector magazines. In the damp climate of South West England they grew fungus and accumulated a lot of dust. Now I keep them in plastic slide boxes, about fifteen of those to an airtight container with a bag of silica gel inside.
It doesn't take long to load slides into magazines and is a small price to pay for longevity.