New Multisite or Domain Access project?

I'm starting to flesh out some ideas on a new project, and I wanted to get a feel on which way to start planning it and what CMS to go for. As Drupal offers Multisite and Domain Access, I'm not sure which would be best suited to my needs. Let me explain what I'm looking at building.

• example.com would be a central domain.
• product1.example.com would be a subdomain focusing on a single porduct
• product2.example.com would be another subdomain, themed differently, focusing on a different product
• Additional subdomains would be created for further products.
• In the future, domains such as product1.com may be obtained to use alongside product1.example.com. The functionality would be unchanged, but the domain name must be preserved.

Each product page would use similar modules, but could potentially have modules that are unique to them. Furthermore, each product subdomain could inherit relevant content from the parent example.com (such as some news articles.)

User registration would be the same across all sites, with full SSO. If a user wishes, they should be able to see all content that they've created across all sites. User messaging should also work from site to site. Also (and this is the really tricky bit that might need some custom code), each site needs to allow users to create personas specific to that product, and to be able to post content (forum posts etc) as that persona, rather than as that registered user.

The focus is for communities to be created around particular products, but also make it very easy for someone to move from one product to another. And while each subdomain (product1.example.com etc) would be tailored to a particular product, the parent site example.com should provide more of a general view.

So, is this something that's doable? What kind of approach would you suggest? Are there any modules that I should consider? Would drupal 6 or 7 be better (with the work on Drupal 8, I would prefer to use 7). Any other snags that I haven't thought of?

Thanks very much for your time, and any advice!