That's a tough decision for you to make. I honestly don't know what i'd do if I were you in that situation. On one hand i'd like to think i'd be honest and not sell my integrity short for an extra phone. On the other hand, Google is so filthy rich that losing one Nexus 5 wouldn't be a huge hit for them. Especially considering THEY sent it to YOU and it's not like you walked out of a store with the intent to take something you didn't pay for.
I however, do not think I could ever really enjoy my phone because i'd thik about that every time I used it. I think i'd send it back and consider it an investment for good karma.