If this double was made for you and fitted of course you will find that mounting the rifle brings the sights into perfect alignment and shots will be instinctively accurate. Then a red dot may actually detract from that perfect situation because you will need to raise your head, albeit a little. If the double is off the shelf or pre-owned then you may get very lucky and be a perfect fit, but if it isn't your next best choice would be to fit a red dot, and like
@Philip Glass I like the Kahles Helia. It is small and looks right on a double, but with nice big window and side loading battery.
If the double is particularly good and you plan to hold onto it forever you may consider getting it adjusted for perfect fit.
A scope would be better suited for a smaller calibre double with longer range capability and expectation, but of course you will now be lifting your head much further and the special feeling of being at one with the wood and metal is out the window.