1 BLOG, 2 templates

I am trying to find a solution to enable a single BLOG instance on my site to be displayed using two different templates, depending on where the user is coming from. I am not looking for a different theme/template for one page…I am looking for the whole BLOG presentation to change.

(1) I want one theme available to the public that has a wrapper around the regular BLOG content containing a link that says something like, “Want to find out if this place is recommended?” This provides a link to purchase the online service.

(2) For people who have already subscribed to online service and go to BLOG after they log in, they should get a different template that does not have the “purchase” text/link and will have a completely different look and feel.

Maybe is there a way to have the public theme use regular URLs and then add a theme parameter to the URL to present the subscriber’s templates (i.e. http://www.domain.com/blog?theme=paid)? Are there other tricks that you can think of and/or recommend?

Much appreciated. And hope this is clear.