To give an example of what studs can do for you I rode in this morning, arriving not 20 minutes ago. The road was wet but the temperature was -1*C and I'm sure most of the road was not icy because of the layer of salt on it but I'm sure there were sections where it was icy. The conditions are right for that type of black ice. But with my Marathon studs I couldn't tell and there was no incident.

The route includes a short park trail as well and that was icy. Without studs I wouldn't not have been able to ride through it.

As for swapping tires/wheels, I have two sets of wheels for the bike. It takes two minutes to swap with a minor brake adjustment. But when in doubt always go with studs.