Both types of searches have advantages and disadvantages. In general, when using keyword searching you will get more hits returned, but some of them will not be very relevant to your topic. However, keyword searching uses natural language and takes less time.
To illustrate the difference between the two methods, try this: click on Academic Search Premier on the library research databases page. Enter the keyword "AIDs" in the search box. You get over 60,000 hits!!! Now, scroll down to item #8 and click on the title to open the record detail. See where the "Subject Terms" are listed? If I really want information on teaching aids, not acquired immune deficiency disease, my search will be much easier now that I know the subject heading.
Now, try this with your topic. Begin your information search with a keyword that you pick from your thesis statement. If, after entering it in a database search box, you get too many hits, click on the title of one that you feel is pretty relevant to what you're looking for. Scroll down to where it lists "Subjects" or "Subject Headings". If you then enter that in the search box and perform a subject search, you will get good results.
Often the most effective way to search is by subject. The trick is in discovering which subject heading to use. When you take the time to do this, however, you will end up being able to zero in on more relevant articles than would be returned in a keyword search.