Having played both routes I kinda prefer the space route. I don't really like the Gaia Sabers as an enemy and those goddamn water stages are too annoying. Space route also ends with a great boss battle with Toki wo Koete as the BGM (both old and remixed versions playing)
In Space you acquire Mekibos, Shuu, Saphine and Monica and the 3 Guest commanders also join you as you fight Zezenan. On Earth you only gain new attacks for Exexbein and Raioh, you also end up fighting the Boosted Childred and Murata a few times - enemies I never quite liked.