As much as I liked seeing Monument Valley, I'm with kale73 . We spent a couple nights at GCNP at the Thunderbird Lodge, so right on the rim for sunrise and sunset. We spent on night at MV in a premium cabin near The View. I'd take GCNP every time. MV was nice to see, and I'm not sorry we visited, but it wasn't somewhere I'll make an effort to return to. Honestly, the Moki Dugway was way more memorable than MV for my family (and not just because the drive was scary, haha).

As far as hiking, it really sounds like OP might be overestimating what is possible in the limited time mentioned. Hopefully you're reading up on preparations and temperatures to expect for the time frame you'll be visiting. We didn't hike very far down into the canyon as we were there with youngsters and were there during the hottest time of year. We settled for a short walk and soaking in the views.