For a dSLR, there's no difference between a clear filter and a UV filter, since the sensor is basically insensitive to UV wavelengths (unlike film).
The 50mm f/1.2L is one of the lenses that definitely requires a filter to complete the sealing (the other definite ones are the 16-35mm II and the 17-40mm; the ones that definitely do not are the super tele primes; other 'sealed' L lenses, e.g. 70-200 IS, might or might not).
So...UV or clear. The reason to choose one over the other are cost and availability (in some lines, clear filters are more expensive, and UV filters tend to be more widely available).
Personally, I use B+W MRC UV filters. The high end Hoya filters (SHMC, HD) are also good. A cheap filter is a bad idea, IMO.