If you don't want to learn more and better yourself your in the wrong industry.  Always go for more.

Well, as a CWI and Level III UT MT PT, I would say, if you can, go for the ACCP Professional level III, (you'll have to write a procedure as part of the test).  and do take a refresher class. It will put you in contact with people who work in different industries using NDT, and since alot of the questions on my tests were experience based, having their experience, and the experience of your instructor to draw from all week will help with the test.

As NDT seems to be an aging profession, and with demand increasing, 10-15 years from now I would think an experienced level III would do OK.

If you want to work as a level III see if there is a consulting company or inspection company near you where you can gain the practical experience, and learn from others while doing so. You'd be suprised how easy some people make it seem to extract procedures from a specification or design requirement.

If you are training and certifying level II's make sure they know what they are doing. Dont be afraid to hold them back or point out their deficiencies early on, and work with them to overcome them. You should have enough confidence in them to be able to stand behind them without doubt should a situation arise where their abilities are called into question. And this will happen.

Good Luck to you

Greg