Hi! Let me make sure that I understand what you are asking. You want to know the best way to help your current site with rankings? You want to know which is better: to create a brand new site on a new domain and provide a link back to the main site OR buy a new domain and just redirect it to a page on your current? If I understand your question correctly, neither of them will do what you want to do. Reason being, the domain you are sending a link from, is brand new. It doesn't have any "link juice" to pass to your main site. Now, if you could find an aged domain that already has links built to it and has some link juice, then you will accomplish what I believe I hear you saying. The only way to "increase" your rankings is to have other sites with authority link to you in some fashion.
Hope this helps, if not, please let me know, so that we can get this resolved for you.