Defining an option that can only be set in certain circumstances

I have an option that must be left with the default value when a certain flag (another option) is false.

I didn't find any example (let alone documentation) on how to implement this, so I've come up with two ideas:

option-that-errors-out-if-set-when-flag-is-false =
let
  default = if config.some-flag
          then "some default value for when flag is true"
          else "value that should not be changed when flag is false";
in lib.mkOption {
  type = lib.types.str;
  inherit default;
  apply = v: assert assertMsg (config.some-flag || v == default) "Do not set this option unless 'flag' is true";
          v;
};

option-that-ignores-value-when-flag-is-false =
let
  default = if config.some-flag
          then "some default value for when flag is true"
          else "value that should not be changed when flag is false";
in lib.mkOption {
  type = lib.types.str;
  inherit default;
  apply = v: if config.some-flag then v else default;
};

Which one do you think is "best" (cleaner, more idiomatic, etc..)?

Is apply the "right" place to validate options? Should I make a custom type instead? Should I approach this in some different way?