Ok, had almost decided on Door County for a few days after going to the EAA airventure show in Oshkosh, WI. But now thinking could do a stopover in Chicago for a few days (to drive to Oshkosh), the then fly to Boston and drive to Maine. Which would you do, and why?

Maine = lobster, you have to ask? There's a small air show at the Owls Head Transportation Museum in Rockport Maine. We saw one in conjunction with a 50s weekend. Our favorite lobster place in nearby.

I've only been up to Door County a couple times and never to Boston & Maine. BUT--I'd opt for Door County. You can have lake views, walks along the beach, trees as far as the eye can see, drives along country roads, shopping, and crowds (or solitude), in Door County. Check the Wisconsin DNR web-site for more info. Why add the stress of another leg to your trip?
And ....
Door County = Fish Boil. (I'd add a smily face here if I could remember how)

Too late for Door County. I just booked some FF tickets - and we can fly to Green Bay, then from there to Boston, then back from Boston. We crave lobster and no offense, that wins out over a fish boil!! We'll drive to ME from Boston. Maybe we can do a day trip from Green Bay to Door County and if really love it, can go another time.

Enjoy your time in Oshkosh! EAA is an exciting time for everyone.
I usually plan a few days of yard work at DS so I can "work" under the western airport approach.
Watching the War birds is especially emotional.