Depends on your setup, both UV and O3 can be totally useless to marginally beneficial. Tell us more about your setup, and I will gladly offer my slightly biased opinion to you.
It is a 29 gallon-reef. No external filtration, 4 power heads operated with red-sea wavemaker, 40 lbs of live rock, live sand as bottom substrate, sea clone skimmer in sump and 250-watt metal halide pendant.