Nine mile canyon is easily worth an entire weekend.
To get the best out of it, you need to really camp there.
It's a long way from Price to the petroglyph area and it's a challenge to drive back in the dark.
You can camp or rent a cabin here http://www.ninemilecanyon.com/
That said, I don't know if many people enjoy petroglyphs enough to spend time in Nine Miles canyon
vs the Arches/Canyonlands area. Most people that are not retired have to set priorites vacation wise.