Which to grind

Do this:
1. Make a plan on what items in the event shop you really want
2. Plan out servant comps that optimally have 5 servants in each available spot with a trait that gets +1 of whatever element is being farmed
3. See which of these 6 comps is weak and which are strong. If none of the 6 are strong do the mountain one with a mixed team.

If you don't have strong teams for the individual instances just keep running the mountain one till you can buy all 4 of the event shop CE copies. Once you have those 4, run the primeval forest for food and water and switch to individual runs (hopefully you'll have developed your respective teams by then)

Tldr: If you can make teams for them run individual runs period. If you can't: run the mountain with a mixed team till you have all 4 event CEs. Then switch to individual element runs. Don't touch the primeval forest till after.

I think each individually is more fast than farm mountain, if u have the servants of course.