Unless you buy a cheap receiver cover mount, an optic is a good investment. I prefer a side mount that positions the optic over the receiver in a conventional position, because it allows the use of a Red Dot or a conventional scope.
Having just said all that
My Norinco AK-47 has a gas tube mount and a holosight.
It's a darn good set-up, but the side mount and 1X-4X scope on my AK-74 is also a great setup.
Ya really can't go wrong with either. Just avoid those top cover replacement mounts like the plague.