I-35W south can be packed on summer Sunday nights. If you drive, allow for traffic delays. It's normally about a two and a half hour drive without traffic, roughly 165 miles with most of the way posted at 70 mph. The good news is that there doesn't seem to be much construction this summer: just south of Duluth (with all lanes open on weekends?) and then around I-694 and along I-35E if you go that way.

DLH has a nice new terminal but as you probably know, it's not real close to downtown and the water, nor is it located along the route to the Twin Cities. I've never been airside and I don't know about airport concessions or lines. With airport security, I don't think flying would save much if any time normally, plus you'd have MSP airport parking fees or airport transportation costs.

Have you figured out where you're going to stay and how you'll get to the airshow? Taxi? Hotel shuttle? Unless you rent a car, what would you do with bags during the air show? There are some hotels in the area along the airport and shopping malls, but nothing would be walkable IIRC. At least one downtown hotel provides local and airport shuttle service upon demand as well as free parking.