I currently have an option to buy one of these two. (I already got insurance quotes and they are both about the same) Assuming the BMW has about 150k on it and the Mustang has 90k Which one would you rather buy and why?
Considering the mileage on both of the cars, what you should be asking yourself is whether you plan on selling these cars at some point in the future or not. If you think you will be reselling in 3-5 years, then definitely get the mustang. You will get much more return out of the mustang at 150,000+ miles at that point than you will out of the BMW with 220,000+ miles. However, if this is a car you plan to keep for a very long time and keep in the family, then I would suggest the BMW. The 3 series is the most solid car in the BMW line-up, and arguably in the entire luxury brand segment of vehicles.
In short, if you plan on reselling in a few years, Mustang. If not, BMW. Good luck!