Originally the manta dive was done off what is now the Sheraton. In June of '99 they disappeared for several months. They were found up off the airport in roughly late August/early September of that year and dive operators were soon finding the airport site had bigger numbers of mantas than the Keauhou site. For the next year the show sort of revolved back and forth between the two sites and then the Kona Surf shut down in summer of '00 and that was pretty much the end of reliable sitings off that site 'til the Sheraton opened a few years later and had their lights on the water for 4 or 5 months.
Currently there are a few large snorkel companies and a dive company that's pretty much a snorkel company these days based out of Keauhou that stick strictly to that site. Pretty much all the other operators head to the airport site (unless it's been unproductive for a couple nights in a row), it tends to have more mantas (when you hear of numbers of over 6-8 it's almost always off the airport site, the south site is usually good for 1-4 though), is more protected from swells and currents, and is much better set up for moorings and handling several boats.
The airport site is definitely a better daytime site (probably among the best dive sites in the state), and generally a better night dive, although the south site is better at night than it used to be as the boulders are getting a bit of coral growth back since they're not being bombarded by anchors on a nightly basis these days. I frankly could not imagine requesting the south site over the airport site as a dive unless the mantas just haven't been showing up at the airport site.
On the dive itself, everyone sits together at the dive site. You can literally have 10 boats of divers and snorkelers within a circle with a 25-30' radius off the airport site. Because of the lack of organized moorings at the south site, on a crowded night it can be a bit of a zoo and things get more spread out. Having all the divers and snorkelers in one small area is actually a good thing on this dive as it concentrates the light... and the plankton which the mantas feed on.
By the way, we did the manta dive on Monday night, it's kind of slow and there were only 5 boats out.... 21ish mantas at the site that evening. We've got another planned for tomorrow night, I'm hoping the big numbers of mantas keeps up - it's been pretty reliable with good numbers since a dead spell back in August.
I hope this helps.