i'm about to mount new marathon plus tires, what is the advantage of presta tubes over schrader?

Tubes should match the rims.

ON touring bikes and MTBs, I only use Schrader, because replacements are easier to find, and some gas stations still have pumps, though there is a presta adapter you can use.

From a cycling perspective everything about Presta is better. Lighter, fits all rims (grommet may be required), easier to attach pump, easier to pump, less stem leaks in my experience, pump can simply be pushed on, no screwing or latching, less weakening of rim, some rims are too narrow for Schrader, nicer look, etc... Unfortunately the billions of cars, and the valve they use overshadows the benefits of presta.