I live in Wisconsin primarily but will be working and living in Minnesota (MN) for the summer. I bought a car from a friend and want to transfer the title to my name. The car has a MN title and plates. Should I register and transfer the title in MN since I will be living here for 3 months or should I try to do it online in Wisconsin since that is my primary home?
You will not be in Minnesota long enough to be classified as a resident of the state, if they follow the conventions of most other states using 180 days of residency as the rule of thumb. I assume you're not selling your home in Wisconsin, nor are you trading in your Wisconsin driver's license for a Minnesota one (correct?). Finally, you're going back to Wisconsin when the job's over, right? So, if you register the car in Minnesota now, you'll just be transferring the title again in three months to Wisconsin when you move back home in order to get Wisconsin plates. To me, the answer is to register the car in Wisconsin. To address the point about insurance raised by another contributor, be absolutely honest with the insurance company about your situation. I'm positive your situation is not novel or unique, so they'll have the right solution for your insurance coverage if you just tell them the truth about the circumstances.
I don't have any legal citation to back this answer up, but if you have a permanent residence in Wisconsin and you aren't going to be staying in Minnesota long enough to get a Minnesota driver's license, then just register the car in Wisconsin.
According to this site, the insurance company cares about where you park your car and not being truthful about it can constitute insurance fraud. Thus, it would seem to me that the address where the car is parked overnight determines the state of registration.
Register the car in Wisconsin. Registering it where you work instead of your permanent residence can be constituted as fraud. This recently happened where I live as people work in my state but commonly live in other states. They were all given tickets and told to change their registration.