When you use a subject heading search versus a keyword search is a judgment call. In most effective search strategies, you will end up using both. Which one is better to start with may depend upon whether you are searching an online catalog or an electronic index as well as the broad subject area of your topic. The more accurately you have described your topic the more likely it is that some of the terms you have identified are also subject headings. In general, if you do not know what the subject headings are for your topic, you can use a keyword search to find a few relevant items. Then use the subject headings on those items to find more relevant materials.
If you are searching an online catalog and your topic is in the sciences, it is often better to start with a subject search on the main term describing your topic because many technical terms and scientific concepts are also subject headings.
If you are searching an online catalog and your topic is in the social sciences or humanities, using a keyword search to find appropriate subject headings may be a more efficient strategy.
If you are searching an electronic index, use a keyword search on the most descriptive terms you have identified to find a few relevant items. Then look at the subject headings or descriptors on those items to find relevant subject headings or descriptors.
If you are searching an electronic index, you need to use a combination of terms whether they are subject headings or keywords.