From your signature you should know this which I know you probally do but here it is. The referring of one IP to another IP. A common example is when you type in one Internet Address, and another Address almost immediately appears. I have never tested it before but I would say that b would have the better rank no matter what.
For SE all dobermans are separate also any domain with and without www treating as separate. So Which get good SERP based on the SEO , but advising to use unique domain while promoting