Personally, I am in favour of a liberal use of site collections. In short, when in doubt, or unless I have a good reason for creating a sub-site, my default choice is a new site collection.
Having site collections really sets IT up with greater freedom to manage and scale the service down the road, whereas sub-sites are generally more limiting. For example, sub-sites all exist in the same site collection container and if two sub-sites grow beyond a manageable size for a single site collection, that container cannot scale out its storage without breaking the collection apart into several site collections and facing other challenges (like changing URLs or having managed path complexity, and data migration efforts).