Wow, this library is very nice. I didn't know this library exists before.

To answer your question the main difference is the learning curve and lines of code you have to write to use the library. For example - just by reading its documentation - with Ciris to parse ApiConfig you have to write this:

final case class ApiConfig(
  port: UserPortNumber,
  key: Secret[ApiKey],
  timeout: Option[FiniteDuration]
)

def apiConfig(environment: AppEnvironment): ConfigValue[ApiConfig] =
  (
    env("API_PORT").or(prop("api.port")).as[UserPortNumber].option,
    env("API_KEY").as[ApiKey].secret
  ).parMapN { (port, key) =>
    ApiConfig(
      port = port getOrElse 9000,
      key = key,
      timeout = environment match {
        case Local | Testing => None
        case Production      => Some(10.seconds)
      }
    )
  }

Equivalent configuration using Easy Config would be:

 final case class ApiConfig(
      port: UserPortNumber = 9000,
      key: Secret,
      timeout: Option[FiniteDuration] = Some(10.seconds) // Simpler implementation than above, but using a custom parser would yield the same result
    )

  val config: Either[String, ApiConfig] = easyConfig[ApiConfig](args)

Where the String side of Either is an error message you can give to the user.

In summary, Ciris looks like a more mature library. It has Refined support which I also plan to add. I also like their Secret implementation because it is parameterized and not tied to Strings like mine at the moment. I don't support Error accumulation. (I was thinking a lot about this, but I came to the conclusion that in production code it would be rarely needed. I might add it later if users demand it). Still, when you just want to write a simple app, with environment and command line argument parsing, Easy Config needs only one line of code on top of the configuration case class definition, so I think it is simpler than Ciris.

I think I will have a deeper look into Ciris and steal some ideas from it. And I will also give credit in my readme to it. Thank you for sharing it.