For 'classic' cantilever brakes, are cartridge brake shoes better than non-cartridge ones?
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.