Both are good and fill a different niche in a reef tank.
As you probably already know, amphipods are much larger than copepods, and are therefore much easier to spot. Copepods generally avoid clean glass, so let the glass get a thin layer of algae and I'm guessing you'll see a lot more copepods. Then imagine that kind of coverage on the rock, substrate, and other surfaces in your tank.
Over time you'll probably see fluctuations in these populations based on tank conditions.
Aren't the little organisms living in our reefs fascinating? Sometimes I find as much joy watching the refugium as I do the DT.
If you are interested in learning about reefkeeping or have a problem with your reef, our reefkeeping community is here to help. Feel free to ask a question or search our site. We have lots of experienced reefkeepers that are willing to provide free reefkeeping advice!
Besides being a great resource for all levels of reef aquarium hobbyists, CaptiveReefs is a social experience that will enhance your enjoyment of reefkeeping. CaptiveReefs is committed to connecting reefkeepers with the support and information they need to grow beautiful coral reef aquariums.