And there's the rub. It's nearly impossible for me to tell you what YOU'd enjoy more. Either trip would be perfectly enjoyable, and I suspect that you already have a preference for one trip over the other, so go with that. For a full up travel day, with only minimal stops along the way, 500-550 miles is a reasonable distance. If you plan on taking short hikes at stops en route or poking around side roads, 350-400 miles would be more in order. Here are links to a number of discussions of those two general itineraries to give you some ideas. Other than near the major parks (Yosemite and the Grand Canyon) on weekends I would think that you can manage quite reasonably without reservations. Most motels require 24 hours notice to allow cancelation without a penalty and even that would cut into any spontaneity that you're trying to maintain.

AZBuck