Okay, for farming ?-7 vs ?-8:

The chests on both maps yield 21 Sealstones a piece.
?-7 costs 30 stamina, and has 4 relatively easy to RNG chests, so 84 Sealstones total
?-8 costs 40 stamina, and has 4 relatively easy to RNG chests and 1 chest which requires 3 nodes in a row to align correctly with a max of 3 tries, so count on 3-4 chests and maybe 1 extra once every blue moon.. So 105 Sealstones total.

So, if you want my opinion: only do ?-8 until you three-medal it, then move back to ?-7 for farming.