Well, depends on what you want.
Heathrow Pros: Very busy, awesome shopping, heaven for spotters wanting to see exotic airlines.
Heathro Cons: Very busy, chances of delays much higher, guranteed time spent in a holding pattern on the way in, crowded, switching terminals a pain.
Zurich Pros: Smaller Airport, not congested, easy to find your way around.
Zurich Cons: Smaller Airport, not as much shopping/amenities, not as many exotic aircraft.
If you want a quick and painless transfer, I'd recommend Zurich. If you want to see the busiest international airport in the world and possibly some exotic planes go for Heathrow. Personally on a flight that long, I'd opt for Zurich just for the sake of simplicity ...