I'm on board with the more empty space position.
Here's why -
Why is a wasteland a wasteland? It should have empty spaces, and lots of them, right?
Additionally, there's the questions of environmental atmosphere. Some atmospheric moods can be fudged a bit with visuals and mood music in the background along with some ambient noises, but, trekking through a large "empty" area could build on that atmosphere, and the environment itself could pose hazards without the need for random enemy encounters. Getting caught out in the open with an approaching radioactive dust storm with no shelter in sight, could present a real danger. Same for radioactive acid rain. What if walking through a swampy area presented sink holes, quick sand, deadly choking clouds of CO2 in low lying areas?
Empty spaces could be enemies in and of themselves; enemies that can only be defeated by surviving them, and only surviving them by planning ahead and having the proper gear. You don't cross a desert without water, right?
I think big empty spaces could be quite excellent, and inexpensive on the computations needed to supply them because it's the land itself trying to kill the character passively, and through the player character's own incaution and poor planning. They be true wastelands in the respect that nothing lives there, but, PCs would have incentive to cross them for what lies beyond, and, possibly finding some loot on the bodies of other incautious travellers killed by the wasteland itself.