Get/Join/App vs. .CO domains

PG's post this week had me thinking about domains & company names. Let's say you want to start a company called "Unicorn". Obviously, you would love to own "Unicorn.com", however, the owner wants $100k+ and you don't have any funding yet.

What's the better option in your opinion? GetUnicorn.com JoinUnicorn.com UnicornApp.com

OR

Unicorn.co