They definitely are either inspired or its probably the other way around, but they do have the same gameplay aspect.
The only difference is mostly on the enemies in the game, since Inside has spores while Echo has weird penis aliens.
From my take of Parasite Echo compared to Parasite Inside, Echo feels more focused on actual combat, puzzle, and survival, because its made like a Resident Evil style game in space, while Parasite inside feels like a walking simulator at most because you mostly walk and not fight as much.
(And im excluding the boss fight from Parasite Inside because its short and thats the only real combat you get from it)
Plus, Parasite Echo is thinking about making grab animations to the game, so its already giving promise of more content worth then what Parasite Inside has made, since that game has not even made much progress in total.
Yeah, the dev of Parasite Inside is just catering to vanilla content, and there is no real monster action or anything much related to parasites happening to the Protag.
Like, 90% of the sex content from there is just about being sprinkled in spore dust and get send to an area tied to a scene (especially if you played the previous versions), and its just pure vanilla (basically no actual cum scene or kinky stuff)
The only thing they did better in the current version is adding some minor hazards that you have to either struggle spam or shoot down, but its very much tame since the one you have to struggle just to get out of does nothing sexy and its only damaging and causing spore toxic.
I actually believe that Parasite Echo is going to get better at it during the next couple of updates.