I thought I would put my two cents in as well. I can't speak from personal experience about these programs, but hopefully, I have something useful to add.
First, U of Chicago is considered by most if not all to be a top 10 rad onc program, and is in a level above the other area Chicago programs in terms of national reputation. Its faculty and conferences are often regarded as some of the best in the country. I consider it one of the rad onc power houses along with programs like MSKCC, MD Anderson, Harvard, Michigan, UCSF, etc. Does this really translate into making it a better residency? Not necessarily, but it certainly doesn't hurt at all.
I was in your position when I was an MS III trying to decide on a lab and a research mentor. I can tell you about some of the things that went into my decision. First you need somebody who is going to be a good mentor number one. This does not mean that you have to work with the biggest name in Chicago, but you should steer clear of relative unknowns. You want to work with someone who is nationally known. This way your letter of rec will carry much more weight when you apply (It most likely will be your most important letter as this person will know you very well). Secondly, you need to get published. From an application point of view, if you don't get at least an abstract out of the experience then you are wasting your time. Preferably, you want to be first author. This will boost your application significantly. I would recommend mentioning that you would like a project on which you could be first author when looking for a lab. This may help you later on down the road when it comes time to submit your research for publication. Authorship can get very political at times. One concern about working with junior faculty is that it could be more difficult to be first author. They often work with a senior faculty whom they often feel obligated to place as the senior author, and they are typically first author. You could easily be bumbed to second author regardless of how much work you to do on the project ( I have seen this happen several times). On the other hand, I know some junior faculty who make it easy for med students to be first author because this allows them to get good cheap help for their research. Just have some idea about this before you choose a lab.
I also spoke with residents at programs near my medical school on who they thought would be the best person with whom I should work. I got down to two or three names with this alone and it was the smartest move I made. Residents know the general qualities you will need in a mentor, and more importantly they know rad onc, the polictics of the program, and they know these people much better than you do. Originally, I wanted to do research with the biggest name at my med school's program, but the residents discouraged this. They said he was way too busy to mentor residents let alone med stundents and that I would be compeletly on my own if my project hit a dip in the road (most do at one time or another). A good mentor will be there for you every step of the way, but especially so when your project feel as if it is going nowhere. A good mentor will write an outstanding letter of rec for you. A good mentor will be your number one advisor and ally when you apply. A great mentor will get on the phone and make calls to get you in somewhere.
You also want a project that can be completed a few months before you apply so that you can submit an abstract. If you work on a major clinical study, it could take years to accumulate the data. Not such a bad thing if you were an MS I, but not so helpful for an MS III. There are plenty of basic science, physics projects, and clinical research projects that can be completed in l2 months or less. As for project selection as a medical student, I personally think it is better to do a less groundbreaking project thas a great chance of leading to publication than working on a sexier project which has a greater risk of not getting published. You will look so much better having a first author abstract on the world's most boring rad onc project than talking about why your unpublished research on the succesor to IMRT called APRT (short for Almost Published Research Topic) during your interview. Programs are looking for individuals that have taken the iniative to do research, developed a project, completed a project, and published their results. The reality of research projects is that beauty is typically in the eye of the beholder anyway. Meaning that whatever project you decide on will bore to death 90% of the people who read your application. We all like to think that our projects are special and ground-breaking, but most research is simply about making mundane and incremental contributions to the field.
You would also like your project to really give you an inside track at the program where you do it. Specifically, will doing research at U Chicago be enough to get you in there. It will certainly help you, but will the program seriously consider someone without a Ph.D. for their program. I don't have the answer to that, but I can tell you the following. Last year UCSF, Stanford, MD Anderson, and Harvard had a combined 17 positions in rad onc. 8 of those spots went to women (one of whom had a Ph.D.), the other nine spots went to guys. Of the men, seven were M.D.-Ph.D. and two were regular M.D.'s who had extensive reseach publications and experience (Howard Hughes Fellow, several first author papers, presentations at ASTRO, from prestigous medical schools, etc.). The point is that it can be very difficult to get into some of the very top programs (of which U Chicago is one) without a Ph.D. these days or at least lots of research. If you are doing basic science research at U Chicago, then you might be even in a more difficult position because they will have their pick of many top-notch superstar M.D.-Ph.D.'s with major basic science experience to choose from when you apply. It can be very difficult to stand out in this crowd. Even if the program really likes you it can be difficult to justify choosing an M.D. with one year of research experince with an abstract or two when they will have M.D.-Ph.D. applicants with multiple first author publications in journals such as nature, science, cell, etc. Working with a great mentor at a place like Northwestern may give you all of the benefits of U Chicago for buffing up your application and also boost your chances at program where you may not need a Ph.D. to get in.
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