i found it useful to start with my ndt cert, allowing for the time required to get my cwi, then got my api,icc(formaly icbo). i did not really ever consider getting a nace cert, due to the lack of work where i want to live. it would be nice to have, but in my area the cost would not be justified.
I respectfully differ with the opinion of the ndt certs. NACE and API 653 are good money makers, but NDT MT,PT,"UT", and RT film interpretation +CWI and welding certs has made me lots of $$$. I don't know about spending more nights at home in bed either. I've been on the road 4/5th of my career, and at times gone in terms of years. I now work third party, and with that I find the knowledge of welding and NDT has paid lots of $$$.
Of the certs I have and have had, UT and the CWI has made the most cash, especially the UT. If your an independent TPI you may want to do some market research into the numbers of petrochem companies who are or are looking into requiring TPI of UT. I realize that sounds bad, but the real life truth is, there are just not many good manual UT hands out there. Take note of recent inclusion of UT in lieu of RT (ASME code case 2235-9 and Appendix U of API 620 to name a few), and the requirements that it be recorded. Food for thought.