"... but I thought it wise to also invite in-depth experiences by seasoned users."
I think truly seasoned users of both systems are going to be rare. I am actually an experienced Drupal developer who has recently been tasked with maintaining a Moodle installation. So my experience is heavy on the Drupal and light on the Moodle. These are my thoughts for what they are worth:
The best thing about Drupal is it's community. It's active and gigantic. There are tons of excellent modules that add bits of functionality that you can use to build just about anything you can imagine. This makes Drupal super flexible. If the module you need does not exist, development is easy and straightforward, the API is well documented and there is a ton of learning material to consume. Dev learning curve is steep but climbable and you can probably hire a developer with Drupal experience tomorrow. But you basically get very little out of the box. You get a first rate permission system, a flexible content model, an OK block system and a decent theming system. You have to build everything else out of contributed modules and your own custom modules.
The best thing about Moodle is that it is an LMS full of LMS features that you didn't even know you needed. Instructors, Students, Grades, Courses, Assessments, granular notification system, and more LMS stuff than I even know about are baked in and you get them as soon as you install it. That is all stuff you have to build yourself with Drupal. But if you find that you need some development to meet your needs, developers are harder to come by and the learning curve is steep. Learning materials are fewer and worst of all, Moodle still uses an old API for lot's of stuff which means that there are situations where you need to do something and the only way to accomplish it is to hack Moode. This is never the case with Drupal.
"So, yes, lots of open content--> think Drupal, but also all kinds of exercises and teacher needs--> think Moodle. There's our 'dilemma'..."
Yeah it all comes down to how those weigh against each other. If the exercise and teacher stuff is pretty simple, then build it with Drupal. The more complicated that is, the better Moodle looks.