For 'classic' cantilever brakes, are cartridge brake shoes better than non-cartridge ones? What are the advantages of cartridge brake pads compared to non-cartridge ones? Do they offer better braking characteristics?

I'm about to replace brake pads on a 1994 MTB which has Shimano STX brakes (front) and STX-RC brakes (rear), model BR-MC33. Both are 'classic' low-profile cantilever brakes. The STX-RC brake has cartridge brake shoes (type M64/T for aluminium rims), whereas the older STX brake has non-cartridge ones. 

Stores still sell Shimano non-cartridge replacement shoes for 'classic' cantilever brakes, but the replacement inserts I need are almost nowhere to find, anymore. I wonder if it's better to replace the brake shoes completely with cartridge ones from some alternative manufacturer, or to just go with easy-to-find non-cartridge replacements.
There's little difference between the two types as far as braking performance is concerned, as you can easily find the same pad compound in both types. I've been able to easily find Salmon Kool Stop brake shoes that fit the vintage Mafac brakes on my Peugeot 12 speed. That being said, the one issue you may run into is the lack of the correct inserts, which might be the case for your brakes. Even if you are able to find them, you may not have much variety. 

Overall, you should use the non-cartridge pads. They're easier to find, relatively inexpensive and should offer you a wide selection. As a side note, you don't need to buy Shimano replacement pads. All that matters is how the brake pads fit into the brake arms.