Surely if you purchase a domain ( and redirect it to your existing domain ( you expect to loose all ranking for If 301's are used then the objective is simply to pass on authority, not dominate the search results with 2 domains. If you want to keep in the index then you would take Rebecca's option 2 or 3.

I had two domains, a .com and .net with godaddy. I’m beyond their redemption period ($80 fee), but they said it should be released back to the public domain soon and I may be able to get it back. Before I knew this, I paid $12.99 plus ican fees for the .com thinking it was available as the rep told me she was able to add it their cart, and I paid a $24.95 fee for back ordering my .net domain. Shortly thereafter, I received an email from godaddy saying:
Thousands of domain names expire every day. The reasons are different. Some owners forgot to renew the domains, some just don't want them anymore or they moved on to other projects. For most people these so called Expired Domains don't have any value. They just see a bunch of Domain Names someone else deleted and move on, but for the people who know about SEO or the value of good Backlinks, Expired Domain Names are money just waiting to get picked up from the street. The only problem is to separate the good ones from the bad ones. That is where comes into play.