From an SEO and link popularit standpoint you will only have to worry about a single domain and all of the link building you do will benefit the site over all. With subs you need to treat each sub as a unique URL from a link popularity standpoint making your SEO efforts that much more work.
From a marketing perspective again directories feel like the better way to go. If you are building a single brand and not differentiating the products greatly other than by subject matter then directories again feel like a better way to go.
Generally I recommend subs for areas of a site that you don't want to have indexed and don't want to worry about natural search for.
As far as unique domains unless you want to operate a network of sites you would just be creating a lot more work for yourself than is probably worth it in the begining.