The requirements for CCDP are:
1. 640-863 DESGN(CCDA)
2. 642-901 BSCI(CCDP)
3. 642-812 BCMSN(CCDP)
4. 642-871 ARCH(CCDP)
After completing these four exams you end up with two additional certs, a CCDA, and a CCDP, and you command a higher salary premium.
My questions are:
1. Why would anyone want to do a CCNP any more?
2. Which would be better preperation for the big cahuna, a CCIE R&S and/or Security?
overall, the number of exams is the same. i think cisco designed each cert with very clear objectives in mind. a ccdp would be designing networks, and a ccnp would be administering them.
Keep in mind with salary that there are many factors that go into that. Also, someone doing design is more likely to be travelling, so they will get a high salary than someone who is fixed geographically (generally speaking). It's not what about a CCDP could do versus a CCNP, it's simply what's in the exams that separates them. I'm sure you will find CCNP's doing network design, just as there are probably CCDP's doing net admin stuff. Not to mention the number of people who hold both.
Having a CCDP instead of a CCNP won't automagically command a higher salary. There are other things to consider. Also, because one engineer is good at design, doesn't mean they'll be good at net admin, or vice versa.
And i'm guessing you've already seen this, but on the off chance you haven't.
http://tcpmag.com/salarysurveys/ -> US salary stats