I: Wish You Were Here

They both have a teenage girl being turned into a weapon to save people, they both have romance and drama, but Zaion contain less romance. 

They both seem pretty similar in the design of the armor. 

Both have settings where Earth is threatened by a hostile life-form, along with enhanced soldiers sent to fight against the creatures, and a girl having the power to ensure the survival of the human race. 

Both about teenagers in small Mech-like super suites fighting humanoid, bloodthirsty monsters. Similar animation and aesthetics. 

A meteor fall to Earth, with a virus that transform infected people into monstera. to fight them, there's an organization with armored people. Also, angsty protagonist that doesn't like what he's doing. 

Zaion or I: Wish You Were Here & Scared Rider Xechs both have the same setting base on some apocalyptic future where humankind in at odds with some kinda of monsters that is either alien or people being effected by a virus. Also there is that one character that serves a special purpose for the team fighting against those creatures that they may be immune or have a hidden power. Did I mention both also have that transformation to fight those inter-dimensional beings so there's that. 

They both have a teenage girl being turned into a weapon to save people, they both have a spirit like thing and the setting is kinda like a futuristic theme one more then the other also one is more darker. 

Both have leading men that are supposed to be amazing but are truly annoying while the stories - seemingly original - are very predictable.  

Similar feeling, the earth have been infected with monsters in Blue Gender and a virus in Zaion which turn the user into a monster. The two main characters in both anime have a similar relationship too as they are mostly treated as tool. I think Zaion have better romance but Blue Gender is overall better because the serie is longer and have more solid content. 

Both of these anime similarities is so much a like and that is that they can tranform and fight monsters but in Zaion its a disease and they fight the creatures in a nanotech suit while in Zetman its more like generic