Should I install this mod or BZ noble metals?

Well, it depends.

Them Thar Hills only adds gold. BZ noble metals adds gold, but also silver, palladium and platinum. If you enable both at the same time, I think the only reasonable assumption is that you want my version of gold, but the other noble metals will be left alone (mostly.)

The metals in BZ noble metals are more realistically rare/precious and only unlocked after blue science, while Them Thar Hills gold is only slightly rarer than other resources and still demanded in bulk. One BZ gold plate is enough gold for five blue circuits, while one blue circuit costs fourteen TTH gold plates (this can change if other mods modify red circuit recipes, like say BZ chlorine). Running both at the same time might therefore end up feeling a bit incongruous, shipping around multiple trainfuls of gold while having silver mostly as a low-throughput byproduct.

BZ noble metals puts an additional kink in the works: with Space Exploration it makes gold ore not spawn on Nauvis. This is a problem when gold is needed for red circuits!

Therefore, if all three of Them Thar Hills, BZ Noble Metals, and Space Exploration are installed together, it will be possible to obtain small amounts of whichever noble metals are enabled on Nauvis. However, you won't be able to gather enough to build all the toys you want. Almost all of your gold/platinum/palladium will have to go into research and Space Stuff until you can import the resources you need from offworld.

Why did you make this when BZ noble metals exists?

I was finishing up the mod when Brevven released Noble Metals. I had wanted to make somewhat more integrated and less clobber-y compatibility between them, but he went on hiatus shortly afterward and we were never able to discuss it. I'm releasing it now because I'm not sure whether Brevven will return to factorio.

Will you ever name one of your mods something normal?

No.