Which is better re: aged domain?

I have a domain that used to rank top of google for about 10 keywords. Since then its been steadily dropping off despite all efforts.


I was thinking about buying a 10 year old domain and just mod_rewrite all the urls to the 10 year old domain.

Would that be better than just buying the 10 year old domain and 301'ng to my domain that exists right now that is slowly falling off?