A similarity thesis is along the same lines as a compare and contrast thesis. In both cases, you are likely to be making a two sided statement. The first side can be stating that two things are similar followed by the contrasting viewpoint. A good thesis will do several things. It will make a statement that is arguable. Stating that the Civil War started in April of 1861 is not a good thesis statement. That's fact. Stating that the Civil War was a good thing for the health of the United States government is an arguable statement that you will be attempting to prove in the body of your report/essay/paper.
A good thesis statement will also typically identify a counter opinion to what you are trying to argue. The word "although" is a great way to make sure your thesis statement does this. For example: "Although cloning technology has greatly improved over the last decade, its success rates are still far too low for human trials." This kind of thesis statement requires you to discuss how cloning technology has developed while at the same time requiring you to point out its weaknesses.
So for your similarity thesis maybe go with something like:
"Although they bear some differences, the similarities between ______ and ______ are quite dramatic."
That thesis will force you to acknowledge the differences to your reader, but also allow you to argue that the similarities are more pronounced and important.