The USCCB version looks good! It's useful to be able to link to a specific paragraph (even though there's no exposed interface for it) and the full-text search seems speedy.
It would be even better if they hyperlinked the Scriptural and other documents referenced in the footnotes (so instead of popping up "LG 7" it would say "Lumen Gentium 7" and link to the actual text), but perhaps that will come in time.
I think the only advantage of the Vatican version (given that it and the USCCB site are just as "official") is that they have multiple language editions. Christianity.SE is in English so I don't think this issue has too much weight; people who are not so comfortable with English might like to cross-check with a version in their own language, but that would be the case for many sources we cite. If for some reason it was necessary to compare different language versions, talk about the use of a specific Latin word, etc., then the Vatican site is always available. Otherwise the USCCB version seems to me to be the best.