It has more to do with which way you want your career to go. If you plan on inspecting in the Oil and Gas industry, then go with 1104. If you plan on working in the structural world, then D1.1
Remember though, API 1104 does not recognize AWS CWI certification. So if you think taking the test with 1104 will help you in securing a pipeline inspection job, well that is not the way it works. May not be fair, but that is just the way it goes.
The intent of the CWI program is to test the inspectors knowledge of welding and the ability to use basic inspection tools and accept and reject welds to a standard. It comes down to what you have worked in. I took my test to 1104 because that is what I am famillar with. I believe had I spent the time to study D1.1 I would have passed the test using it.
And when you pass the test, remember it is just the beginning. In addition to my CWI I have and maintain 5 additional certifications and have in the past had to train and qualify in MT, PT and UT. The CWI is only the beginning.
When I started inspecting full time I had a welding rig and all the farkles and sparklys. I wanted to be able to weld and inspect.
Be sorta versitle.
The best advice I ever received "If you are going to weld, weld. If you are going to inspect, inspect. You cannot be the best at both. You can be good at both but never the best. And what are you going to do when you get laid off a inspecting job and go looking for a welding job with a contractor you inspected over the week before." I sold all the sparklys and farlkes. Kept my SA200 for about 5 years, then sold it. And I have never looked back.
Both careers can be frustrating or rewarding. Just depends on you.
I would research pipeline integrity as a career path. With welding experience, a CWI and two to three years as a gypsy with a PI company, you can write your own ticket. If you have the drive and desire.