SnakeX

I have about 10 blogs that dont have unique content of them.I was using sponsor rss feeds on them.
They are indexed at google but they dont have good ranking.
I added last moth unique content on them and keep posting.
I want your opinion...what is best to do?
delete all old no unique content and keep only the last good seo friendly posts?
or
Keep the old rss posts and keep posting new unique content until google loves the site again?