The biggest difference between your options is the ability to switch back and fourth. There are many benefits to a cantilever for a scope on a slug gun, especially if you aren't planning on dedicating it to one single function.
My shotgun goes from turkey with a saddle, to a rifled barrel with a cantilever to a bunny/bird gun with no attachments. None of these require sighting the gun back in during a swap (I always do though), and none leave any trace of a mounting system. I can swap back and fourth in under a minute.
A rail is fairly permanent, it absolutely can come on and off but that will involve wear. I would suspect that the scope would have to come off the rail each time to access the screws, that is no fun either.
If you are dedicating the gun to one purpose a rail may be in order, if you have any intention of utilizing the gun you would be better off with a mount that can be swapped easily and quickly.