Systematic Reviews usually focus on a clinical topic and answer a specific question. Extensive literature searches are conducted to identify studies with sound methodology. The studies are reviewed, assessed, and summarized according to the predetermined criteria of the review question. Meta-Analysis takes the systematic review a step further by using statistical techniques which summarizes the results of several studies into a single estimate of their combined result. Practice Guidelines are systematically developed statements to assist practitioner and patient make decisions about appropriate health care for specific clinical circumstances.
Guidelines review and evaluate the evidence and then make explicit recommendations for practice.
The pyramid serves as a guideline to the hierarchy of evidence available. You may not always find the best level of evidence to answer your question. In the absence of the best evidence, you then need to consider moving down the pyramid to other types of studies. It is a therapy question and the best evidence would be randomized controlled clinical trials comparing these 2 drugs.