I usually pronounce "URL" like a word, rhymes with "hurl". Most of my tech geek friends do too, probably because it's the shortest pronunciation. That means I say "an URL". But if you pronounce "URL" "You Are Ell", then it becomes "a URL". Which is more common?
These are rough estimates, but it gives you an idea. Too bad search engines are case sensitive: curious the difference between "URL" and "url".