Certification in Instructional Designing

If you're looking for a credential to wave around, consider ASTD's (sorry, ATD's) Certified Professional in Learning and Performance Certification (CPLP).

Personally, I would be very careful about more degree work, you could end up spending a lot of time and money for not much return.

Build a business case for yourself that weighs the time and money cost of whatever it is you're thinking about doing versus the return you reasonably expect to get from it.

Candidly, if I were going to be hiring an instructional designer, I'd be more interested in seeing work samples and satisfying myself that the person can actually get things done.

I was once asked in a job interview the difference between formative and summative evaluation, and my response was "I used to know the answer to that, but I'm a doer, not a theoretician, so I'm sorry I'd have to look the terms up" I then discussed different methods of developing and evaluating training and ended up getting the job anyway.