As with everything here, it's up to you - That's the point of a roadTrip rather than a tour or cruise where somebody else determines your itinerary. All anyone here can do is tell you what they would do. For myself, I'd pick Walnut Canyon, and in fact did so when I had the same choice you're facing. Waterfalls are cool and all that, but after about five minutes they can get pretty boring. Besides, in August in the desert, there's no guarantee that the falls will be 'spectacular' enough to warrant even those five minutes. Walnut Canyon, on the other hand, is a nice hike, some great scenery, and the ruins of an ancient culture all rolled into one. Be sure to stop in at the tourist/ranger station to get suggestions for what would best suit your interests.