The PDF prices of D6 Space, D6 Adventure, and D6 Fantasy are great! Is there any advantage to one of the versus the others?
Yes, but it's pretty subtle... D6 Space was originally slated to be Star Wars 3rd edition... but LFL pulled their license. It retains the WEG Star Wars force powers, relabeled as psionics. Adventure is the same core, but with different attribute and skill names and a few different options as standard; it has a build your spell magic system and a second (different from Space) Psionics system.
Fantasy has a similar (If not the same) magic system as Adventure, but has a religion/miracle system, and some more attribute and skill name changes and differences.
All have a wound level option and a hit points option.
all three have the same base vehicle rules, but different vehicles in the lists...
Space is a star wars derived list of equipment.
Adventure is a modern list.
Fantasy is, of course, faux medieval.
There are just enough options to be customizable in the rulebooks, and by mix-n-match of the paranormal abilities across the three, and attribute mix-n-match, you can tweak the feel of the attributes while making no substantial mechanical changes.
Space feels a touch more complete, but it's more about tech and tone. And they are fully interchangeable mechanically; the names were changed for feel, not substance. Pick for desired feel - Space is great for SW, doable for Trek or Orville (with some new template creation). Adventure is for moderns and Indiana Jones, and can do horror as well. Fantasy is... not Herc and Xena.
(H&Z was a variant using only whole dice... )