Mazinkaiser

"Modern" OVA's of classic Super Robot anime's that define the entire genre. Getter Robo and Mazinger Z both get a "current" day anime style animation with "Real Robot" (if you are familiar with that genre) elements thrown in. Thus creating bad ass sequels to both classic Super Robot series that set the bar for everything Mecha anime.  

Both are modernization "alternate timelines" of souped up versions of classic Super Robots. Mazinkaiser is the more modern and beautifully rendered series while Shin Getter Robo has a grittier artwork and pacing that I prefer so even though this is a recommendation section, I would suggest not taking the similarities much to heart. Treat this more like two similar series but one having more flash and the other having more substance. Shin has the substance, Mazinkaiser has the flash. ...but don't forget...these are still remakes/sequels of classic Super Robot series with depths that may disappoint more modern anime fans so make the flash and substance issues relative to how those classic Super Robot series were and not a general list of modern mature anime in general. read more 

Both are Go Nagai series which are OVA sequels to their 1970's anime counterparts. New Cutey Honey makes occasional references with Mazinger Z lore and features cameos from other Go Nagai series. These OVA's contain violent action and maintain a similar tone as their original anime. 

Like Shin Mazinger, this is another continuity reboot wihich is closer to the original toei tv animation than the manga.  

Both shows share the same basic concepts, famous characters and giant robots. The Mazinkaiser itself is a new, different mecha and everything happens within seven episodes. Shin Mazinger presents a fully alternative story that starts from the beginning, after showing us a taste of what's to come in its first episode, which can be enjoyed on its own. Mazinkaiser is also based on Go Nagai's Mazinger Z but it is aimed at a more mature audience, assuming a certain familiarity with the original material that isn't required by Shin Mazinger.  

Both have Super Robot hotblooded Super Robot pilots with spikey hair that can kick serious ass. Kouji and Domon both summon their respective mecha with a pose/snapping their figures and a piece of dialogue.  

On the surface both shows feature über over the top mecha and hot blooded pilots. The mechs even share many similar abilities. (Not surprising since Gurren Lagann was an homage to 70's mecha anime which is where Mazinkaiser has it's roots). They also share some of the same type of stylization in design and animation. Mazinkaiser sticks to it's 1970's roots but does feature a heavy think lined art style much like Gurren Lagann. Both titles have some oddly kinetic and bouncy animation and are very colorful.  

There's a famous pic lying around on 4chan /m/ that makes the connection between godannar and mazinkaizer. also, tatsuya shares the same voice actor with kouji in Mazinkaiser. both also contain hot blooded songs by Ichiro Mizuki and some really good fanservice. I can't help to think that Godannar was very much inspired by nagai's 70s mecha series. i may be wrong, but that's the general gist i get from watching it.