The adventurous and kind-hearted Van is a 17-year-old boy who lives in a small village on Planet Zi – a world where giant beast-like mecha known as Zoids live alongside humans. One day, while out on his hoverboard, Van is chased by a group of bandits, and ultimately finds himself in the midst of some old ruins. To his surprise, he finds a hidden passage that leads him to pods containing a wild Zoid (which he names Zeke), and an amnesiac girl named Fiona. To protect his new friends from the bandits that lurk around each corner and to help Fiona regain her memory, Van decides to travel alongside them; and thus their adventures begin!
The universe of the future is divided between the Earth Alliance and ZAFT. After a year of war, ZAFT attacks the neutral colony Heliopolis to steal five prototype mobile suits. The mission is a success, but a young man named Kira stumbles upon the fifth Gundam, and he may be the Alliance's only hope...
Both of these series do share a great deal. Both comprise of wars on a global scale fought utilizing giant robots. While Zoids' storyline may seem slightly less mature and distant, taking place on a planet other than Earth, it's characters seem a bit more unique than SEED's were, so the two seem pretty even in that regard. In short, if you like a series with great mech action, you'll find it in both.