Between the forum or blog, which tool is the best for driving traffic to you web site?

It all depends on the mission of the site of course. Both blogs and forums are great tools for marketing and both can be used and can benefit your site enormously. A blog requires one or a few authors to log in, but you have complete freedom as to what you say and show visitors. A forum will get you a lot of extra traffic, but it will also get you a lot more spam and flaming, which would need to be moderated and controlled. This takes a lot of time and effort.

If you do not moderate your forum, or only moderate the forum every so often, then you will need to begin to establish a reputation within the forum before you "pitch" your product or service. I can say from my experience, however, forums can be a great bargain for nice sponsorship opportunities. Starting out with a good forum script, you can add custom areas, display banners, text ads, or just talk about your own products or services.

Blogs, on the other hand, tend to have a much tighter area of influence. If people work with the blogger and develop a relationship, it is likely that you can promote your service or products in conjunction with the blogger's message. However, the downside is that most blogs don't have much of a reader base, unless you've found one of the few that get fair amounts of traffic.

Many people nowadays are using the forum or blog format for promoting products. Over the past few years blogs have been used more a promotional tool whereas it was only a means of communication, a journal if you will, before. Most bloggers (writers) can more freely say and promote what they like, use, or sell.

Regardless of the two online marketing formats, be sure to research and find which works right for your business needs. A good rule is to install one of the more popular ones like Wordpress for the blog or vBulletin for the forum since they are more widely supported. phpBB is also a good choice if you're hosting or running your site using PHP.