Personally, I don't use either on a reef - just live rock and a protein skimmer (and maybe run some carbon periodically).
Many don't use UVs anymore, but I think the jury is still out - they do some good things and some bad things. You can certainly have a great reef without one.
Canister filters work fine, but I think most folks avoid them for a reef because they should be cleaned frequently to keep from collecting crud and fouling your water quality (ie. too much maintenance). Used periodically or cleaned frequently, IMO they're OK.
If you already have one, personally, I would use it for periodic detritus removal (blast the rockwork with a powerhead or turkey baster while running the canister) and/or running some carbon from time to time.
