I'm not sure if certain mobs have better drop rate or if it's all the same, but assuming that it basically is the same for each mob and stages have no drop modifiers, then aiming for stages with more mobs is the better choice for exchange mats.
I haven't been focusing too much on farming exchange items rather just leveling up. I did run 7-2 a few times though for some exchange items since I was close to exchanging the eidolon again. It has 7 or 9 enemies as you might end up encountering a single rare enemy during stage 2 instead of 3 enemies. I don't think the rare one drops any of the items needed as it's a dark unit but I'm not 100% sure. It seems to be the only stage with a chance of 9 enemies in the 2nd zone.
For the 3rd zone, a quick glance at the jp wiki, 14-2 has 9 enemies while 15-4 only has 7. Of course 15-4 gives better exp so there's that to account for.
Anyone with more knowledge of the games drop system is free to correct me. There's also Free Quests which I haven't bothered to look through but I do believe they drop exchange mats too.