Skimmer, Hands Down. The skimmer will remove dissolved organics which harbor ammonia, nitrites, etc. UV will kill free floating algea and virius/bacteria. Id say that you will more likely want to prevent the first rather than the latter.
IMO...It's not really a "vs." issue between the two....they perform two different functions and are "complimentary" and not "mutually exclusive"...
However, if it's a matter of cash flow (ie: which should I get first ?) I would favor the Skimmer over the UV as giving more "immediate" benifits in the reduction of organics in the tank and support of the biological filtration...
squid bro.. u put down perfectly on how u cant compare which is more essential... u can do with-out either.. the uv.. u can qt prior to adding to the main.. the skimmer.. u can do water changes.. if i had a choice. id get a good skimmer....but if i was to either ger a crapper skimmer (aka berlin..) or a uv.. id get a uv..
My choice would be to go with the skimmer. Many folks regard the UV as an option of questionable utility although I run one myself. Skimmer has always been considered as an essential part of most saltwater systems for many of the reasons given above.
John