I am happy to help with your thesis statements for these two novels (and add a few ideas on body paragraphs as well). First, let us discuss what an argumentative essay assignment is asking for, and then we can get into the thesis statements for the two novels.
An argumentative essay is really another way of asking for a persuasive essay. Do not get into the habit of thinking that you are supposed to write in an “argumentative” or abrasive way. Instead, you are suppose to suggest an idea (or “argument”) and then support your position. Perfection would be convincing the reader to choose your argument over any other because you have used appropriate quotations from your primary source and facts from either primary or secondary sources. In regards to a five-paragraph argumentative essay, I have my students write the following paragraphs: introduction, body paragraph 1 (opposition paragraph disproving the opposite opinion), body paragraph 2, body paragraph 3, and conclusion.
In regards to In Our Time, you have to first keep in mind that this is a collection of stories (Hemingway favorites, most of them). The collection includes the Nick Adams stories such as "Indian Camp," "The Doctor and the Doctor's Wife," "The Three Day Blow," and "The Battler." They are all about very complex emotions. I would suggest basing your persuasive/argumentative argument around the main theme that connects the stories: death. The following might be a good argumentative essay thesis: Even though one might be tempted to choose family issues as the main thread of these stories, the real crux of complex emotions centers around death and its inevitability. In your opposition paragraph you should discuss the temptation of being led only into family issues. You could use "The Doctor and the Doctor's Wife" as an example (because of the tension between Nick’s mom and dad and Nick’s eventual choice of siding with his father). Choose any other stories for your body paragraphs. All of them are good examples. In my opinion, one body paragraph should focus on Indian Camp and how death as an inevitability is an imperative theme there. Obviously, the doctor saves a baby’s life while the father commits suicide: a perfect example of the moroseness of the circle of life.
In regards to A Farewell to Arms, because this is not a collection of stories, it is a bit easier to create a persuasive/argumentative argument for a thesis. The irony is that you could almost use the exact same thesis above for A Farewell to Arms; however, this one might be more appropriate: Even though one might be tempted to name this an autobiography of Hemingway, it would be more appropriate to highlight the central tension of Hemingway’s famous novel as the futile effort of love in times of war and tragedy. For your opposition paragraph, it will be good to note how Hemingway’s real story did have an effect on A Farewell to Arms. After being denied admittance into the army, Hemingway does experience the horrors of war on the European battlefield. For your body paragraphs, however, you should concentrate on the characters of Lieutenant Henry, an American, and Catherine Barkley, a nurse from England. They begin an affair together, of course, and as they experience the horrors of war, they attempt to ignite their love even to the point of desertion on the part of Henry.
In conclusion, you seem to indicate through your question that you need to write two different essays (and not some kind of comparison and contrast essay involving both books). If this is not the case, then you could repost your question asking for a comparison and contrast essay idea (as opposed to argumentative essay ideas).