I try to read dieseltruckresource.com a good bit and keep reading about people with the 6.7 coming into the dealer with DPF issues and weird codes, sometimes as soon as 1k miles. And these are completely stock trucks!!! 
I don't claim to be a diesel expert at all, it just seems to be common sense to get the 5.9. And make sure you are comfortable with the payments! Don't forgot insurance on a brand new vehicle will be higher too. 
