The year is Future Century 60; much of mankind now lives in colonies surrounding the earth. After being ravaged by countless wars, every four years the dominance over the colonies is decided in a martial arts tournament called the Gundam Fight. This year marks the 13th tournament, and Gundam Fighters representing their colonies have begun to gather. Their purpose is to fight, and fight, until only one remains to claim the title "Gundam of Gundams." But Domon Kasshu, Neo Japan’s representative, has another secret mission; and should he fail, all of mankind may pay the price. Gundam Fight! Ready! Go!
Both G Gundam and Shin Getter Robo vs Neo Getter Robo have ridiculous (-ly awesome) mechas that fight epic battles all while shouting attack names to save the world from a dark force. Both series also feature a mecha horse (!!) and both display countries other than Japan in relatively entertaining ways. Also, the villians in both are pretty crazy; in G Gundam it's an evil Gundam, in Shin Getter Robo vs. Neo Getter Robo, it's dinosaurs who acquired alien technology.
I feel as though G Gundam definitely is the better overall work, mainly because it is long enough to sustain story and gives time for character development, but Shin Getter Robo vs. Neo Getter Robo is an extremely fun and entertaining work coming from the one and only Go Nagai. Give one a shot if you enjoyed the other!