I'm an applying to programs this year as well, and have been admitted to KF with a 75% scholarship. While I have not yet been admitted to any of the higher ranked schools I applied to yet, I have thought over those "what if" scenarios regarding the more prestigious schools. When I think about these hard choices here is a couple things I keep in mind that may help you make your choice.
The first thing I did, which is very beneficial if you having yet done already, is to frame what "B-School Success" looks like for you. For some people, this may truly be MBB or bust. This may be especially true for individuals with very high paying jobs already or with very high aspirations. However, I suspect many people say "MBB or Bust" without analyzing if that is what they consider b-school success for them (note: I have no desire to go into consulting so am not as familiar with what all the options are outside of MBB and what they entail). However, what your B-School Success looks like may be very different -- it could be to have a more rewarding career, to switch roles, to move to the Untied States, to make over X dollars a year, or any mixture of these or other ideas.
Why is setting these goals important? Because I think people, especially in the MBA process, become victim of what I call "aspiration creep". Don't get me wrong, there is NOTHING wrong with setting high goals for oneself. However, one enters the process saying "I want to switch industries to consulting", which becomes quickly, "But only at these top level firms", to "Only at MBB", to "Only at the most highly sought after offices of MBB". Likewise with schools, people can quickly move from "I want to get into a Top 20 school" to "Only top 5 for me!" very quickly. This is why before you spend time thinking over your choice, you should take a half day to figure out what success looks like for you. The risk otherwise is, to put this in a real world scenario, to walk into a car dealership with the full intention of buying a family sedan for $25k with what you need, 4 seats, A/C, a radio, and a sunroof, and slowly but surely get talked into a BMW 7 Series with massaging seats and a champagne cooler for 100k+ (which they bought on a larger loan since they only budgeted for the 25k family sedan). By setting your goal now for B-School Success you can better determine the best choice for you. Because just like some need a family sedan, some might really need that BMW 7 Series if they are planning on going to Monte Carlo to gamble tonight!
In sum, define b-school success from the personal and professional side for you, look at all the opportunities, and define what will best get you there.
The second part is to really spend some time to think about what prestige means to you. As no matter what, even if you determined UNC and Kellogg can equally lead to your personal B-School Success, you will know you didn't go to the more prestigious school at the time and in some circles people may be aware of the prestige gap. However, the VAST majority of people will not know whether one school is more prestigious than another. Most people would be aware that UNC and Northwestern are very good schools and that you probably are a very smart lady. I think there is a value in prestige, but it is far lesser than what the average person attributes to it.
Those are the two things I'm am spending time thinking about now. I apologize for this long post. However, I have one more quick anecdote that might be beneficial. I made a friend moving into my new apartment building who is a resident physician in a highly, highly sought after field. We were discussing one day her life over drinks and she mentioned how she turned down med school at a very, very prestigious medical school and instead moved across the country to a flagship state university, which is far less prestigious. After a little prying she admitted she did it as the state university gave her a full ride. I asked her if she was happy with her choice, and she said absolutely. She discovered a new place, made great friends, and most importantly of all, she knew she wanted to be in this competitive medical field and was able to leverage all these extra opportunities to make her dream come true. She now is a doctor, with 0 debt, doing her dream job. She has 0 regrets. Nobody I know thinks she is any less intelligent, less impressive, or less accomplished for what she did. Whether you chose Kellogg or UNC, you will still be you. You will probably have great experiences either way. The average graduate of either of these schools is in the Top 5% of US Single Income Earners.
Best of luck and I hope you make it to the admitted students day at KF! I'll be there!