Security? using vbulletin is like leaving all your windows and doors open with a big sign outside saying come takke all my stuff
I choose Xenforo and it comes as no surprise as vbulletin has failed how many times in a row? the other day when it changed its support forums to the new beta just to revert it back to the old one after errors were splurting out everywhere and it was impossible to post or read anything.. i LOL'D
I would go with IP.Board for my experience with it, i only just heard of XenForo a couple of days ago
I like IPB Look and they would be my first option if I ever put money on my forums. The free option is phpBB forums. I don't know much about forums, but MyBB seems a bit too simple to me and seems to shrink avatars. i don't like that. But i like the quick edit. Zeta boards took away some browsing options I like like when seeing a member profile and vBulletin is not that good any more to justify its fat price. XenForo sounds great but I don't finish liking its default theme, yet it loads very fast and using it is really smooth.
I choose IPB first and foremost. I tried Vbulletin but that admin panel is crap. Xenforo is good but it lacks features and doesn't feel like a full software. But i use IPB on my main site and XF on a site i jsut started as I don't really care if it gets big right now and XF lack of features arent that important for that site.