Neo Ranga

Similar in a broad sense of being mecha shows, and in a specific sense of centering around tribal beliefs/gods on southeast Asian islands. Ancient 'spiritual' mecha are discovered on these islands, and outside factions militarily vie for control of them contrary to the wishes of the original tribespeople. 

Both follow trios of girls who are related to each other, as their lives are changed by the sudden introduction of the series titular mechas (if you can count Neo Ranga himself as a mecha). Ranga is often more serious but not without it's lighthearted moments, while Ariel is more laid back overall. Ranga is however much more fleshed out, as Ariel feels more like an extended promo for the novels or a failed pilot episode 

both have similar concept of a giant living mecha that is also an ancient god. both dance around such topics as environmental issues. 

Both are written by Sho Aikawa, and are very thematically similar - a fight for Japan, corruption in the higher echelons of society, a country destroying itself from within - and even have suspiciously similarly named military organisations with somewhat questionable ethics. Aikawa's writing style is distinct, and the shows carry a very similar atmosphere and pace, as well as being 15 minutes an episode. Ranga is far, far superior to Rouran, however. 

Both are about seemingly normal people finding a powerful monster/thing and causing problems for other townsfolk.