Both versions are great. However, you need to know few things.
vBulletin 3 is a lot easier when it comes to customization. With a little HTML/CSS knowledge you can make the template look like you wish. There are lots of mods out, but for your own safety, you shouldn't use a lot of mods, just those that are necessary like: Image resizers.
But, vBulletin 4 is something different. It's still in development, there is no stable release yet, all of the current releases are some sort of "BETA". As there are bunch of bugs that aren't fixed. vBulletin 4 is more for the simple boards, for the forums of news portals, business boards, etc. Those users who don't want more, older people who like simplicity.
There are also lots of templates for vBulletin 4, but I only prefer one, the default template. As It's really professional and sexy. You need to know also that vBulletin 4 uses completely different template structure, you'll need to read manual from vBulletin.com to understand few things about vBulletin 4 customization.
If I were you, I'd go for vBulletin 3, but get yourself XAMPP and install vB3 & vB4 on it. Play around with the options, test them on localhost. Before actually releasing your forum.