What you are experiencing is the vaccum advance and the difference between ported and manifold vacuum at the vacuum advance. Some cars will drive better with the vacuum advance hooked to one or the other, you'll have to drive it both ways to find out.
Now for some theory, at cruise where the mixture tends to be lean, and essentially has a slow burn to it, it needs more advance to burn the mixture efficiently for good road manners and drivability. The total amount of vacuum advance also depends on the combination, some like more vacuum advance, others like less. Make adjustments and drive it to see how it reacts. This is tuning for drivability.
High amounts of advance with the vacuum advance plugged in is normal and will not hurt your engine, as cruise is a low load situation. At and near WOT, the vacuum drops off to 0 and there is no advance brought in from the vacuum advance canister. That is where your mechanical advance has total control over the timing. If you had that level of advance at WOT your would likely be having detonation issues and you would be able to see it on the spark plugs and hear the pinging (provided the car is quiet enough). I've driven my El Camino with its locked out timing on the street and its a little lazy at about a 55 mph cruise. It did drive better with the vacuum advance and the mechanical advance, but it sees so little street time that it hardly matters. Its a bracket car first and it will be what it is on the street.