I will be applying in June to medical school. I am deciding between signing up for either the July 16th or July 30th MCAT. Would I be at a significant disadvantage if I took the July 30th MCAT as opposed to the July 16th?
The earlier the better, but either way you'll be at a disadvantage. There will be people complete at schools before you even take your MCAT, and you'll have to wait another month before you get your scores. Some schools won't release secondaries until they have your MCAT score, so you'll have to have a really fast turn around time to get caught up.
If you apply after getting your MCAT score back, it would take about a month to get your transcripts verified in August, so I'm glad you are submitting your application in June. If you are not sure of your MCAT perfoormance, you might consider applying to just one school initially, and then adding others later when you know how competitive you'll be. It takes only one day for the entire application to be sent to newly added schools.
I don't think two weeks makes much difference at that time in the cycle. You'll need the most optimal score possible to compensate for being somewhat late.