If a person has a "locking" differential there is no difference in driving as long as the vehicle remains straight. If you were to turn your steering wheel either direction with a "locked" differential in the front axle the outside wheel needs to turn faster to have good steering without force being applied to the axle, the outside wheel travels a farther distance to become straight again. The inside wheel of course spins slower and goes less distance. With the wheels being forced to spin at the same speed with a "locked" differential it causes strain on the tires and axles, on hard surface the outside wheel is usually going to squeal unless it has the better traction then the inside will. Also the tires will wear or if the traction is really good with the tires then the axle will snap.
My definition of:
Full-time or AWD: no driver input required to have 4WD, meaning no worries about the wife making a poor judgement and not putting it in 4WD when needed or putting it in 4WD when not needed.
Part time: driver input is required to lock transfer case in and/or turn manual hubs in. Meaning better MPG and less wear on all moving parts. Also means wife will most likely be running 4WD when it is dry and sunny or be in a ditch cause she thought it was already in or didn't think she needed 4WD.
Possibly with this thought as well, part time 4WD might "lock" the differential together with vacuum lines or electronically controlled solenoid for a truer 4WD.