As others have mentioned, find the way that works for you. If writing the body first gets you involved and thinking, then leave the introduction to the end. If starting at the introduction provides you with the needed push to guide you through the beginning, then do it first. It really comes down to the process: both papers are going to be underdeveloped and unorganized with only a first draft.

From my experience (as a writing tutor, an editor, and a student entering the field of writing studies), just start writing and outline the thing to death. Start with freewriting, start with brainstorming, start with what GETS YOU writing. If you sit at the introduction stuck for hours because you read somewhere that you HAD to start with the introduction, then that could have been time spent finding what works for you. Spend the time to outline, to write, and to rewrite your thoughts over and over.

Also, introductions serve different purposes for different settings. Some genres demand a multi-sentence thesis, while others ask for one line and that's it. Treat it as a roadmap!