My vote goes to the Marsupial after using both. I still run the Kuiu, while my brother runs the Marsupial magnet harness.
The Kuiu is a good harness. It works especially well with all their other gear like the hydration bladder covers. I have a Kuiu pack, so having everything integrate with itself is a nice perk.
That being said, I think the Marsupial is better quality and I like the magnet system. The build quality feels better overall. The padded straps are more comfortable than Kuiu’s, although not as slim. The Marsupial harness actually has more accessories too, if you’re not worried about pack integration. Not to mention it’s made and based in the good ole USA.
Personally I would get the no magnet harness purely for possible compass issues, as long as the design works for you.