Aside from the practical differences (program lengths) and given complete fluency in both German and French, which offers the better MBA program, in terms of curriculum, prestige, and important career options?

HEC has a more flexible curriculum, greater prestige and better outcomes.

That's a pretty clear and definitive answer, thank!

I'm also interested in both schools, but leaning to ESMT because I want to end up in Germany (I speak the language fluently).

if you can, aim for a better school in Germany

A better school meaning WHU or Mannheim?

Yes.