Hello, a friend of mine has been put in charge of implementing a world class message board for use for various recurring topics of conversation. This needs to run 24/7, so reliability, ease of use, standard usage and feel by other users is vital and critical.

The community would have ~40,000+ users and of these 50-2000 might be logged on at any one time. It would have high repeat visit usage if implemented correctly, so the ability to track replies to your postings are also critical. The same goes for searchability.

I've been told that three vendors are under consideration:
prospero.com
invisionboard.com
vbulletin.com

If you are familiar with each, could you please rate them in terms of reliability, ease of maintenance, max user issues you are aware of, server load problems, understanding of coolness rating (i.e. what sites that cater to a younger crowd are using), ease changing and moderation, etc.

This will also be a private community with very sensitive data, the history of flaws in security of any of these vendors to hackers trolling for e-mail addresses and things like that are also appreciated. Any other valid reasons you would use one over the other are welcomed.

Thank you so much for your time and assistance, I greatly appreciate it.