Dear S Broomhead,
I would have to say it depends on what you want. The certificate you purchase for being ordained, goes to credit you with being an actual minister. The Doctor of Divinity is an honorary degree but does nothing to prove you have been ordained.
For instance, as a past nurse, I could have many certificates that would prove the advanced knowledge I took in the various fields of nursing, however none of them would allow me to register as a nurse unless I had my graduation certificate of nursing from an accredited school, as well as passing the government exams. The other certificates only furthered my chances at getting a job, as they were not required learning to be a nurse as prescribed by the government curriculum standards.
In other words, one goes to prove your status, the other goes to prove your education. They are completely separate things.
If anything, the Doctor of Divinity would be the superfluous of the two. First, because it proves nothing in way of being a minister, and secondly, it is an honorary title and degree.
Hope this helped to answer your concerns!
God's blessings,
Rev. Campbell.