I would definitely choose Iowa, but that's only because I really, really love corn.
In all seriousness, you're in a great position--congratulations on the admits and scholarship offer! At this stage, I wouldn't worry at all about the cost, since the two are ultimately so similar in your case, and an extra $20,000 is a rounding error when you think about it over the course of your career. And I wouldn't obsess over the average salary data or the rankings, either. You could argue that BYU maybe has a slightly higher profile nationally, but the differences in salary and rankings aren't all that huge.
I'm not an expert on BYU or Iowa's business school, but I have a very positive impression of both places in terms of the quality of the teaching. I don't think you can go too wrong with either place, but I'm willing to wager that you'll encounter substantially different environments at the two schools, and there are definitely differences in the curriculum, culture, and types of firms that recruit on each campus. Honestly, I think you should focus on the qualitative differences, and don't worry so much about the data from US News or Businessweek. Go wherever you think you'll have the more enjoyable learning experience, and you'll be fine.
But maybe it all comes down to whether you want a change of scenery, since you already did your undergrad at BYU? Have you been to Iowa City? It's roughly the same size as Provo, but the similarities pretty much end right there. I like Iowa City, and it's a great place if you want a good, solid, tight-knit, Midwestern-type community. But if you plan to stay in Utah or somewhere else out west, then BYU might make more sense.
Sorry, I'm rambling. Good luck with the decision, and let us know what you decide to do!
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