Which of these two options is more sustainable? By sustainability here I mean which option causes less environmental footprint when producing the leads or the refills respectively.
It is hard to say because there is such a wide variation in ink formulations. Also, no company is going to tell you what is in their ink, as they will consider that a trade secret. That being said, the only consumable part of a mechanical pencil is made entirely of graphite which is 100% Carbon and Clay. Go with the pencil!
Here is a "How it is made" video on Graphite pencil leads
Here is a video on ink making.
Very few people use fountain pens today. It follows that few such pens are made. A good fountain pen lasts a long time if you take good care of it. If you have to choose between Parker and Mont Blanc, go with the former (and the one with the gold tips are rumored to last longer):
You can get one on Amazon for under $200.