It depends. If it is on the top of the nether then it is better. If it is below the nether then it is not better unless you half slab everything in a specific radius.
Overworld farms > Farms under nether. Not sure if they're good above nether as I haven't seen/used one above it.
Let's get a few things out here because at the moment no one has really brought up an argument for either. Overworld farm - Pros / Cons Pros: No slabbing Easy to build, cheap materials No redstone Cons: If you don't have a good computer a lot of portals/particles make lag. With EMC spawning limitations can't be built as big but still can be built large enough to be efficient. Nether Farm - Pros / Cons Pros: High efficiency if you put work into it. You can make designs that also farm the other nether mobs. (magma cubes and ghasts) Cons: It's the nether, I mean the place with all the fire and lava and annoying ghasts? You have to make sure no other mobs can take up mobcap in a perimeter this means slabbing/spawn proofing a large area. Resource heavy, pistons, redstone, lots of stone or other explosive proof block. Feel free to add to this list as it was typed up fairly quickly. I recently built a pretty efficient overworld gold farm and I love it.
Well, my answer was based on amount of gold produced per hour. Though, I wouldn't consider a few double chests of obsidian and lots of build material cheap, though compared to the nether they might be. I'm unsure on how much a nether farm cost.
I have a pretty inefficient gold farm, it's in the nether and I spent lots of time slabbing and building an in the end it wasn't as efficient as I thought it would be. Just save yourself time and make an overworked farm
I personally like the nether ones better, but the overworld ones are more efficient with supplies and gold production. This may be because mine is really slow, but if you make a farm like qwerty189 with 360 portals, you will get a ton of gold
