This is really a matter of preference as stated above by Gus and Mark.
If you're looking for water clarity, I would use GAC in a reef and see if that makes you happy.
However, I'm going to get on my soapbox and get all "preachy".
Let's break down what a UV filter really does...it kills free-floating parasites that can harm fish and it can also lead to better water clarity. As an example, it can help kill free-floating marine ich. However, it cannot kill the parasites if they are already embedded in your fish's gills....killing your fish. A UV filter will help to decrease the spreading of MI to other fish by killing what passes through it. Unfortunately, if you have an ich outbreak you cannot guarantee that 100% of your water volume will pass through the UV filter. There will always be some amount of free-floating parasites that never make it through the UV filter. On the upside, the UV filter will help to reduce the amount of parasite in the water column, thus decreasing the odds that it can spread to other fish.
Now for the preachy part....if you quarantine ALL new fish from the start and treat them for illness before they get into your display tank, then in theory you should NEVER have any parasitic illness in your display tank. If you never have any illness in your tank, then there would be no need to have a UV filter. After learning the hard (and sad) way, I am a big proponent of quarantine tanks.
If water clarity is what you want, then carbon is what you're looking for.
