Believe it or not, the middle of the US is not that flat. And the secondary roads are not that straight. The middle of the US was created by glaciers a long time ago. They came down from the north and left lots of really fertile ground behind. The ice sort of flattened things out when it pushed and receded. But except for a few patches here and there throughout the thousands and thousands of square miles of ground, its not very flat. You might not need to use your inner chainring to get up any hills, but you will have to shift both derailleurs frequently. And you will have plenty of turns during a day of riding. Nothing is very straight. As for dull? I've never felt boredom while riding a bike. Not even commuting to and from work in the dark. Please tell us how this "dull" occurs.
Of your two suggested routes, I would opt for the second. I've seen quite a bit of the US via a car. And a little bit on a bike. Your second option has a diverse array of geography and hits quite a few of the grand scenic sites in the US.