Simon,
Both units are incredible and have their own pros and cons. The biggest difference I see in the two units is simply geography. In the USA, Preston is standard. If you are in Europe, then WCU4 would be more common. I do like that I can control the entire Arri camera with the WCU4 which is something that isn't possible with a preston. At the same time, Arri makes cameras, lights, support equip, etc whereas Preston only makes WFF units.
Rob