I get what you are saying about separating the new content from the sales content but I think I would still go with the sub-directory.
You have well established domain names that Google associates with the well known brand - stick with it. Your new pages will rank for the terms for which they are relevant and when they do, the relevant traffic will go straight to those pages, never having to even see the sales copy unless they choose to click in to it.
That is how I believe I would approach this. New domains require building authority from the ground up - absolutely achievable, we all do it... but when you have an already established, totally relevant domain.... use it.
