With MOSS it is not uncommon for people to have doubts on how to structure sites since there are multiple options and each one offers flexibility in terms of managing different sets of permissions, UI customization etc. Here are my thoughts on this subject.
Lets assume we need a site and start off with a webapplication and a top level site collection. To hold related or detailed information of whats in the top level, I would go ahead and create subsites. Now lets say there is some unrelated info which I need to present. I would look for a different web application if
- I need to specify different permission levels for the site (not permissions)
- I need to have customized web-parts that I would not like to be exposed in my current site
- Changes or maintenance in the new site should not affect other sites (recycle app pool, change web.config parameters etc)
- Different storage capabilites need to be defined for the new site (Recycle bin size, Max No of users, upload capacity etc)
- I want to use a different authentication provider
- Be able to give site collection owners ability to specify if the site can have anonymous users and my initial site does not allow that ; or the other way around
- Start from a different root level as compared to my original site
One point to consider here is that each web applications have associated default jobs with them that run on a schedule and also each app pool consumes a bit of memory. I really would not go for a different web app unless I really needed it.
If none of the above are satisfied then I would look for a different site collection if I need to
- Use variations and set it up differently from my original site
- Have different set of policies eg expiration, auditing etc.
- Have the sites managed by totally two different sets of users but does not fall into the category requiring different webapps
Everywhere else just having different subsites looks like a good option.
I have created a checklist that summarizes the things to consider.
This is not a hard and fast checklist but some things that need to be considered for deciding the site hierarchy.