For a protection filter, dSLRs are basically insensitive to UV wavelengths (unlike film), so there's no difference between UV and clear - get whichever is cheapest.
Hope that helps...
Agree on the fact that dSLRs are insensitive to UV... getting a "UV" filter is more for protecting the front element than anything else now.
On the "cheapest" though... Using ANY filter will degrade IQ, but on better, higher quality filters there is less degradation. If you were to say using the cheaper choice between two similar filters, the only difference being one is just a normal "clear" filter, and the other a "UV" filter, yes, by all means go for the cheaper option, but if one is cheaper because of lower quality/less coatings... you're gonna be sacrificing IQ there.
Of course, taking the filter off when the situation allows it will be good too.
Oh, also, most L lenses "require" a filter to be screwed in to "complete the weather sealing", so something interesting to note while you're outdoors.