I personally like Invision Power Board more, just because it is more user friendly and I find the interface far more attractive. The older versions (1.3x and below) were the only free ones, but they were not very good.
I believe Invision is now up to 2.1.3 and it's looking great! Unfortunately it now does cost money, as does vBulletin. In terms of security, they both have their flaws. You could do a google search for each and there would probably be too many to read.
I also really like vBulletin, but I believe that the newer versions of Invision are really starting to compete with the newer versions of vBulletin. User friendly support is available for FREE for MANY Invision sites, while vBulletin support is far more exclusive. The Admin CP in Invision gives much more control over the site; the one is vBulletin has more options but this is only because they bloat the software with every mod anyone could ever be looking for.
The other good thing about a lifetime Invision license is that it gives free upgrades for EVERY change Invision makes. They have tons of sites full of free skins and mods; even mod requests. I used vBulletin for about a year and couldn't find any of this...
I am not, by any means, saying CF should switch; I like both forum systems. I feel, however, that IPB is really starting to outshine Vb