Unlike Splash vs. ToT where I simply couldn't vote for one or the other, I have no qualms about voting for Horizons.
While some would chalk it up to nostalgia (and I readily admit that nostalgia may play some role in my decision), for me Horizons is superior for these key reasons:
1. It fit better to the central theme of EPCOT Center. Just read the dedication plaque. "Human achievements are celebrated through imagination, wonders of enterprise and concepts of a future that promises new and exciting benefits for all."
2. Horizons was the keystone attraction that tied all the other existing Future World pavilions together.
Spaceship Earth/Communicore = The scene transitions are all the various family members communicating with each other not just across the planet but even through space thanks to the living quarters on the Brava Centauri space station.
The Living Seas = Where TLS's SeaBase Alpha was a deep sea research station, Horizons' Sea Castle was what SeaBase Alpha would have eventually evolved into: an underwater colony featuring all the amenities of a small city.
The Land = Using cutting age farming techniques and technology to produce a crop in a land with little in the way of arable soil, just as the greenhouses found in Living With The Land feature vegetables growing in sand.
Imagination = The Look Back At the Past that showcased differing visions of the future, from Jules Verne to 1950s science fiction.
World of Motion = The evolution of travel from land and sea based automobiles to spacefaring personal shuttles (one effect was supposed to feature a projection of Tom Fitzgerald's submarine morphing into one of the small space vessels featured in the finale, but was scrapped because the transition did not work correctly), beautifully capped off by the Space Shuttle Columbia seen launched on three story tall OMNIMAX screens.
Wonders of Life = A segment of the OMNIMAX video talks about how we are living healthier lives thanks to studying the human genome, featuring a computer-generated breakdown of a dna strand.
Universe of Energy = Different ways of getting energy from the environment including harvesting energy from solar panels as well as from kelp.
Meanwhile, Mission: Space is only broadly dedicated to space. Its focus is Mars, but they can't call it "Mission Mars" because they already had an attraction called "Mission TO Mars" and the imagineers of the time had not yet given up on imagination. (See: "Journey of the Little Mermaid" vs. "Voyage of the Little Mermaid")
(PS - Disneyland now has "Snow White's Enchanted Wish" WHO THE HELL CAME UP WITH THAT NAME?!)
3. Horizons was made for all audiences, which is something that Disney sorely needs more of these days. Throwing roller coasters up all over the place only leads to throwing stomach contents up all over the place.