API 1104 is a welding code primarily relied on for pipeline construction (gas and liquid). There are many API codes that also specify 1104, but in most cases, ASME Section IX is accepted as an alternate.
It's funny, DOT allows both ASME Section IX and API 1104 for pipeline construction, but most owner operators really only allow 1104.
If you intend to inspect only boiler and pressure vessel work, study the ASME code and take the test using that.
Most people that are in the CWI field do not limit what they will inspect, so it is really shortsighted to think you'll be able to get away with understanding only one code.
If you have a company that will pay to send you to the AWS seminar the week before the exam, I suggest taking it using D1.1. Afterall, the seminar is going to cover how to use D1.1, and it will all be fresh in your mind the day of the test. If you have to pay your own way, there are seminars available that cover which ever code you desire. If you are not going to a pretest seminar, you should study the code you are most familiar with in order to be able to pass the test.
In the long run, you will need to understand the basics of all codes. You aren't expected to (and in fact are discouraged) from memorizing codes. SInce they all change frequently, it will only cause you trouble if you start relying on memory. So, instead you need to understand how to look information up in what ever code you happen to be refering to for the task at hand.
If you want to talk this over, send me an email.
Charles.