Have used both for vocals. Prefer Air gigs... a little higher quality vocals, but a little more pricey. A couple of tips: Make sure reference samples on site are of high quality, because if the acoustics/mic is poor in sample that's probably what you'll end up with. #2 Don't use vocalist from foreign country unless you're ok with having a foreign accent on your project. #3 Recommend using vocalist whose samples more closely match your type of music, e.g. don't get someone who specializes in metal to do a ballad. Finally, make it clear if you're ok with a vocalist taking liberties with your melody. This seems to always be a problem - that they interpret the song to how they may like it and not how you send it to them.