Nice script!
I think a good idea would be to use else-if instead. If you use this, then mIRC will automatically skip the stage if it has already found the answer, thus meaning the script doesn't keep looking:

on *:TEXT:!random*:#:{
  if ($2 == number) {
    msg $chan $rand(1,10) %+ $rand(1,10) $+ $rand(1,10)
  }
  elseif ($2 == text) {
    msg $chan $rand(a,z) $+ $rand(a,z) $+ $rand(a,z) $+ $rand(a,z)
  }
  else {
    msg $chan $nick Please select either Number or Word.
  }
}

If anything which is NOT number or text is typed, then the script tells them what to select!