ZRH being a bigger airport, you'll have a better choice (well, better is slightly exagerated
) for an 8 hours long layover. If I were you, I'll decide based on the arrival time at LHR or on the kind of service that you expect on board or, perhaps it makes more sense, on the price of the ticket. Finally, 8 hours may be long enough to make a short journey to either city centre. Have you seen either of them? If yes, choose the other.