In my (somewhat limited) experience, there's two ways you can go:
Tree Farming: Grind Kindred Wilds stage 5. Enter a battle, flee until you find a white Sycamorte Tree (the dark ones aren't worth the time), kill it, rinse and repeat.
I recommend a high damage kin for this, even if it has low health.
[KINDRED] Watermeat costs about 30-40k and at level 50 Bloody Bite always 2 shots the white trees. Watermeat does have a 10% miss chance, so you should patch it with an Accuracy Amulet (that's a Square amulet effect), this saves you a lot of time in the long run.
Trees drop trash sometimes, but it's usually 500-1000 plat, so it's a good way to get gold immediately without waiting for your items to sell.
Capsule Farming: Encounters on Land of Sweets stage 5 have about 60% chance of dropping a capsule, so you just click the stage, beat whatever comes up and repeat until you have a lot of capsules. Open them and sell whatever pull out of the RIGs.
I'm not sure what kin to recommend for capsule farming because the enemies are quite weak and I just use the aforementioned Watermeat, but I think most lvl 50 kin should be able to one shot enemies there.
This method is quite fast, requires little setup and can be more profitable if you're lucky with the RIGs, but it's very tiresome because you have to
open all the capsules and then open every RIG individually, and then you have to wait until they sell, but this is a very good method if you're patient enough or just like opening RIGs :p
What's better? I'm not sure, I think capsules are better overall but when I need quick cash I farm trees.