I run an online shop and I wonder what would be more SEO-friendly URL for a product page:
a) domain.com/category-name/product-name OR
b) domain.com/product-name
I already have URL-s for product category pages with format domain.com/category-name.

On one hand I heard (but cannot find proof for) that Google like tree hierarchies in URL (vote for "a"). On the other hand though longer URL could lead to smaller kewyord density, also "product_name" comes as the last URL part so probably the least important (vote for "b"). Maybe both options are equally SEO-effective?

PS. I know about canonical URL's but this is not the case, I don't want/need both URL's formats, just want to choose the best.