If you are doing a comparison, please state what you hope to accomplish relatively early in the paper. You may also want to have a summary or conclusion paragraph, though this is not always necessary. Be as precise and rigorous in your comments as possible. If you are going to make a generalization, provide one or more supporting references. If you are simply stating your opinion, be clear that that is what you are doing.
Writing is hard, and good writing is especially difficult.
Writing forces a person to work through his or her thoughts. If your thoughts are not clear to yourself, that will be reflected in your writing. I have found that the hardest part of writing is determining precisely what I have to say. In the case of a theorem, if I really understand the result, then the writing is relatively easy. If, on the other hand, my thinking is a bit sloppy, then the writing is painful, because I am forced to apply rigor to my thoughts.