If you want to be do research, then the more time spent doing your PhD (within reason) the better. If you don't want to do research, and just want to get a PhD for the certificate, then don't do it.
So in general, a 5 year PhD >> 3 year PhD. Except that in the US, you spend two years doing coursework and 3 years doing research, so it just depends on how much more stomach you have for doing exams.