Well they are for 2 very different purposes. The UV
sterilizer will kill any living thing that goes through it, be it diatoms, algae, Ich, etc. It will NOT however remove these from the water column. They are designed to run constantly (or a good portion of the day), and the bulbs need to be replaced every so often.
The diatom filter does not damage anything, it simply filters to a very fine level. The benefit here is that, unlike the UV
sterilizer, it will REMOVE the particulates clouding your water, as well as any living thing present in the tank (diatoms, algae, Ich, etc.). They are meant to be a quick polishing of the water column, or remove debris after a tank rearrangement/large pruning of plants, etc.
I do not believe diatom filters are designed to run constantly. They are somewhat loud in operation, do a great job of quickly filtering your water, and are easy enough to disassemble and clean that I love mine.
For a tank of your size I think you'll find either a good investment, as if you have a case of GW or other floating algae the fastest way to get rid of this is using one of these.