1. I'll do the repair either way, so long as the repair fee exceeds the cost of dark matter at the NPC vendor. I think many would prefer gil, simply because dark matter does drop from gathering and dungeon chests. Some players have a steady source of grades of dark matter from sources other than the NPC vendors, so offering more dark matter doesn't help them as much as gil.

2. Someone may have evidence to the contrary, but I don't think it makes a large difference at the moment. I believe it affects crystal drops from gathering, so you may want to skew it if you need significantly more of a particular element. I keep them all even for now, but balance changes may cause me to adjust them later.