Grand Canyon or Southern Utah?
That is a really tough question. You won't ever regret an aerial tour of the Grand Canyon. But to many of us, Zion, Bryce, and the other southern Utah parks are among the very best, most scenic areas that America has to offer. If it were me making the choice, I'd probably go with Utah. But if the idea of the aerial tour is too hard to pass up, then you should do that. I don't think you'd be making a mistake either way. Especially since you would still be including the North Rim, which is a gem all by itself.
While writing this, I tried to think of what the difference is. Among other things, the Utah red rocks are more accessible -- in visiting there, you are right in the middle of them. You can hike amongst them, climb on them, sit on top of some of them, etc. If you have no significant time to hike, the Grand Canyon is more remote and "impersonal." The colors are more subdued, and you will feel like you are looking AT the panorama, rather than being in the middle of it like you are at Bryce, Zion or Capitol Reef.
I would definitely split Mesa Verde and Monument Valley into two days. And by the way, you will really like the Cumbres and Toltec train ride! Bob