Design and implementation/operation at the engineering level are pretty tightly linked. However, if your primary desire is design then I would pursue the CCDP certification-wise. That said, it is useful more with the professional services community as a pre-sales or consulting engineer.
If you're looking more to do the implementation and operations sort of work for your present or future employer then I would say the CCNP DC is more useful.
While you're in a position, learn anything relevant you can about the job(s) at hand while expanding your knowledge in areas that interest you. That will make you an all around better engineer and increase your satisfaction with your professional life. It may involve certification, it may involve academic education or it may just be listening to podcasts during your drive to work.
Finally I'd recommend you to a recent blog post by the esteemed Russ White. Here's a link.
