One thing I have noticed with many web sites and forums in general is that most have some type of portal system in place that bridges their forums to the main page. It can display anything from stats, to latest posts, to articles and much more. On the other hand many other websites have a main page (or pages) and have the forums as an add-on to their site rather than the other way around. So what should you choose for your new site? Well I hope that this guide will help you in deciding which to choose.
First and foremost, you do not need both a portal and main page. The reason is that they both save basically the same purpose: an entry way into the forums. The difference between the two choices is not enough to warrant having both on your site. Having made this disclaimer, I would like to present two situations. Situation one is that you are preparing to build a community for teens to hang out and talk about school and their life. Situation two is that you are building a site for a company that provides a hosting service.
As in situation one, when you are building a community (I stress that word), for teens, the main part of your site is the forums. A community represents interaction between members, and what better way to interact than with forums. But you already know that! I stress the fact that forums are the major part of a community because, in this situation, I would go for a portal due to the fact that forums are a major part of a community. A portal will be a gate way into the forums, and thus the visitor will not see a listing of all the forums and be overwhelmed, but instead will see select tidbits from the forums (hot topics, stats, etc.) that will hopefully entice him or her to join your community. In addition if you feel like adding a system where users can submit articles you can seamlessly integrate such a system into the portal. In summary, portals are best fit for sites that have a large focus on forums because they act as a gate way into the forums that also entices the visitor.
However in situation two, the hosting company will most usually not focus on their forums. The will put much of their focus (besides their servers, etc) into developing their main web page/site. Their goal will be to pull in potential customers via the website. The forums are usually a side thought (usually they are support forums for customers with perhaps a place for potential customers to ask pre-sales questions). In this case the forums are not the major part of the site as it was with situation one. For this company just having an entry way will not suffice. Therefore they put their focus on the web page/site and ignored the portal, unlike situation one.
I hope that by having used two different situations I have explained when to use a portal and when not to and instead go with a web page. In short and summarizing, if you plan to focus on the community and forums, go with a portal to entice your visitors to join your forums. If on the other hand if you wish to have the forums as an addition to your main site, do not use a portal and instead integrate or link your forums from your main page.