Compare-Array.ps1

function Compare-Array
{
  param(
    [array]$Ref,
    [array]$Diff
  )

  $max = [math]::Max($Ref.Length,$Diff.Length)

  for($i = 0; $i -lt $max; $i++){
    if($Ref[$i] -ne $Diff[$i]){
      [pscustomobject]@{ 
        Index = $i
        Left  = $Ref[$i]
        Right = $Diff[$i]
      }
    }
  }
}

Compare-StringSet.ps1

function Compare-StringSet
{
  param(
    [string[]]$Ref,
    [string[]]$Diff,

    [switch]$CaseSensitive
  )

  $Comparer = if($CaseSensitive){
    [System.StringComparer]::InvariantCulture
  }
  else {
    [System.StringComparer]::InvariantCultureIgnoreCase
  }

  $Results = [ordered]@{
    Left = @()
    Both = @()
    Right = @()
  }

  $temp = [System.Collections.Generic.HashSet[string]]::new($Ref, $Comparer)
  $temp.IntersectWith($Diff)
  $Results['Both'] = $temp

  $temp = [System.Collections.Generic.HashSet[string]]::new($Ref,[System.StringComparer]::CurrentCultureIgnoreCase)
  $temp.ExceptWith($Diff)
  $Results['Left'] = $temp

  $temp = [System.Collections.Generic.HashSet[string]]::new($Diff,[System.StringComparer]::CurrentCultureIgnoreCase)
  $temp.ExceptWith($Ref)
  $Results['Right'] = $temp

  return [pscustomobject]$Results
}

@fatherjack I added a Compare-StringSet example as well:

$ref,$dif = @(
  ,@('a','b','c')
  ,@('b','c','d')
)
Compare-StringSet $ref $dif

Output:

Left Both   Right
---- ----   -----
{a}  {b, c} {d}

Fabulous!

Many thanks
J