Comparison/Contrast Essay On "To His Coy Mistress" and "My Last Duchess".

I was given an assignment to write a comparison/contrast essay on 2 poems of my choice out of a selection that we recently read in class. The problem I'm having is that we must include some sort of argument in the essay. I chose these two because I thought the narrators both wanted something from the woman that they are speaking to/of. I'm just not sure if that is the best or easiest argument for this essay. Are there any other ideas for an argument? And, if not, I'm not sure how to word the argument in order to put it into a thesis statement.