Sorry to hear about your old company but congratulations on these new opportunities. It's a great question.
The top line answer is: changing jobs will not hurt your application, unless the job change does not fit into your career narrative or comes out of left field. The important thing is to show that you are actively moving closer to your ultimate career goal. You want to think of your career narrative as an equation: a+b+c=d, where 'a' = all the skills/experiences you've gained to date, 'b' = the skills/experiences you'll gain in bschool, 'c' = the skills/experiences you'll gain in your short term career, and 'd'= your long term career goal. You want to show that the steps you are taking now all work towards "d".
So, as to the question of which job would be 'better', I would say 'it depends'. If the whole reason you are going to bschool in the first place is because you want to ultimately go into finance, or into new product development (both of which would flow logically from the position at Fiat/Chrysler), then going to Fiat makes a lot of sense! If, on the other hand, your intention is to stay within strategy and/or within the steel and mining industry, then perhaps it would make more sense to stay put. The fact is that you have several years within the industry, and a couple solid years within the company, so a switch wouldn't look bad. If it fits with your career narrative, it could in fact improve your application and show that you are someone who takes initiative to move closer to what they want!
Hope that helps and good luck making this decision. Also consider at the end of the day which would make you more happy. If you don't get into business school this year, you could be in the position for longer than you are anticipating, so you should really do whatever feels right for you.