Both of them have mechas and aliens occupying the Earth looking for energy with the purpose of evolving and making Earth their home planet. In my opinion Tekkaman Blade has a much better story and characters but that doesn't make Mospeada look bad or something. They are both cult classic animes worth watching.
both have an alien invasion of beings that are actually parasite's, seeking the best lifeform on the planet to take over and survive.
The only difference is that in tekkeman blade, there is no clear or real origin of the invaders, only that they wandered space, in macross SE 3 ( robotech the new generation) they have a clear and open storyline about the invaders origin, and even in some way that humans can communicate with them.
both are nice and oldskool
Both series involve a world in which society now follows far more basic rules of survival and in which many of the cities are rather broken down. In Mospeada, people can still talk, but similarities between the two are uncanny. Both also involve a group traveling around to different cities and have some similar experiences among them, even if their goals are different. If you want to see more ruined cities and people living in now terrible societies, check the other out.
Both series involve a small group traveling a great distance in order to get to a specific point on the globe inconveniently far from where they started while fighting an alien species that has dominated humanity. This changes halfway through Blue Gender as the focus shifts a bit, but the alien presence on Earth is still unwelcome. Blue Gender is the darker of the two series, but if you enjoyed the traveling a great distance while fighting things that want to kill you aspect, you should consider giving the other one a shot.
Mospeada and Outlaw Star are both presented in an episodic fashion and involve a group of people traveling through their respective worlds(In Outlaw Star, space; Mospeada, North and South America), with an ultimate goal in mind(In Outlaw Star, unlocking the mystery of the Galactic Leyline; Mospeada, forcing the Inbit invaders from Earth). During their travels, they meet with various people and undergo multiple ordeals, while moving towards their goals.
So do you like Fighter jets that transform into Gerwalk and Battroid mode that fight aliens? If so then you'll like both of these series. Both of these series are unrelated in terms of universe but were both adapted into the Robotech universe.