In my humble opinion: the mountain gives better chances of pictures. Even better and important: no glowing moon .. if you want to scope for deep sky objects etc. Because the moon reflects too much light.
Long story copied:
What kinds of areas have better dark sky conditions?
Getting farther way from population centers is key, but there are some other factors that can enhance your stargazing. Being at higher altitude helps, because you'll be above the dense air at low altitudes that contains hazes, fog, and smoke that mask your view of the stars. If you can safely get to a viewing location a couple thousand feet above the surrounding area, it can make for clearer skies.
You don't have to find locations super far from roads or highways, either. As long as there aren't lots of bright lights nearby, you can find decent spots quite close to well-traveled highways. This is because light pollution is a cumulative effect, so it's worse the closer you are to more developed areas.
It also matters which direction you observe. You might have a dome of light pollution from a city 30 to 50 miles away on one horizon, but spin around the opposite direction and you could find a much darker sky. You also might find a great spot for stargazing under a moderately dark sky, with a view that looks toward an area under a significantly darker sky. An example is a mountain campground under Bortle class 4 sky, with great views in the direction of a Bortle class 3 sky over the ocean.
