What link ranks better domain.com or domain.com/category

What I mean is Im planning to market one of my bodybuilding products on its own and create backlinks but dont know if I should create its own landing page or use my existing domain.

What would rank better for the serps a new domain for your keyword or use my existing bodybuilding domain that already has backlinks and create a new page from it.

Example www.bodybuildingcom/bsnnoxplode
Or www.bsnnoxplodecom

If i created 100 of the same backlinks for each of the 2 domains above what would rank better. Would www.bodybuildingcom/bsnnoxplode rank better because the domain www.bodybuildingcom already has links to it or would the wwwbsnnoxplode rank better.

You want to create an indented double listing for more screen real estate so backlink both of them with your desired keyword.