MIBIII is more it's competition. It's an action blockbuster also for the whole family rather than a more mature audience.
PROMETHEUS I think is going to surprise everyone in how it does. Suspense thrillers never do that great at box office. Or I just wasn't around before 'The Sixth Sense' to see them do that great in terms of big numbers. It's a different demographic. I think it will be in the top five movies of the year box office wise, but not how some are expecting. It's the only real suspense thriller this summer so while opening moderately and not posing any real looming threat, it will continually do good throughout the summer because of it's specified narrowed demographic (no competition in it's genre) AND being thought provoking. Basically a repeat of 'The Sixth Sense' which earned 290 mill at box office - in 1999, inflated it would be much more. So I'm guessing around the same amount (if the movie is good).
Basically the way Snow & Prometheus will bring in money will be radically different. It's also extremely different demographics. Two different planes of operation. Snow will be front-loaded like most blockbusters, Prometheus will operate more as a sleeper drawing in crowds outside of the suspense viewers via word of mouth like 'The Sixth Sense' did. As said, despite being seen as huge in fan circles - it's not the usual big draw genre these days; which is why I'm seeing a repeat of what happened with 'The Sixth Sense' (it had August in it's favor, but also inflated it would be more than 290). 'The Sixth Sense' was partly a surprise because of what it did for it's genre (suspense thriller) which doesn't happen often and when it does I'm pretty sure it's always accumulated over time rather than all at once due to it's initial narrowed demographic.