VB is worth the money you shell out for, phpbb due to its fantastic community backing is good choice if you cant shell out the money for VB.
Vbulletin is an excellent board but the fact that PHPBB is free will reduce the 'wealth' feel of your site. It is the embodiment of cheap to be honest. If you are going to go for a free board then i recommend Vanilla. It's a sexy looking message board, free and is more unique in operationally terms over PHPBB which can give your message board a more exclusive feel. The first forum system I used was PHPBB and after I secured a number of users I realised that I had the user base to justify purchasing vBulletin. VB comes with an import tool called Impex which allows you to migrate your PHPBB/Vanilla/PUN/etc.... database into a Vbulletin one. I would first go for a free message board (Vanilla - my choice) and then if your forum is a success get Vbulletin.
phpbb are very easy to be hacked , but we found many succesfull foorums with more than 500.000 members and without any problem ... it is free , and ther are many Mods to dowload vbulleting is more secure and will cose you 160$ . but it is a good software with a pro interface you have the choice
If you can afford vBulleti then go for it. If you're on a tight budget then phpBB is fine to start out with as long as you keep up to date with the patches. Then, if your forum is a success you can upgrade at a later date. VB do provide decent tools to convert other forum DBs over to their software, so your choice doesn't trap you forever.
If you have the money, vbulletin is the way to go. InvisionBoard is real good too (great admin cp), but vbulletin is just a bit cleaner. But if you want free, phpbb is great as long as you install a number of mods, particularly anything that improves the captcha, as the default one is useless. We were completely overrun with bots and SPAM until I installed an anti-spam CP mod and it's been well over a week know without a single bot making it through.
I have vBulletin, it's a great forum but the annual fee is a huge turnoff after paying the initial $160. (you'll have to pay $160 every single time you want to use an additional forum too, there is no discount). I'm looking to make several other forums and am having a hard time deciding on a free one. I like phpbb but I'm afraid about the security, and I've heard nothing but bad things about the pre-RC version that's coming out next. I guess my choice is now between taking a chance with phpBB, SMF (which I know nothing about) and Vanilla, which I heard about for the first time several posts above
I think if you can you should go to phpbb, firstly it is free and more importantly unlike VB they wouldnt shut you down if you run a site that is not acceptable by them. Phpbb 3.0 is great just wait for RC1 which is going to be released soon.