Some random thoughts to help you make up your mind:
1) You indicate home base is Geneva. If correct, then presumably it would be easier for you to fly GVA-LHR-YHZ than to have the long drive/train/added aircraft connection to ZRH.
2) You get significantly more points travelling through YYZ if that is important to you
3) On the odd chance that you have any Air Canada SWU upgrade certificates, the lowest eligible fare from ZRH (H fare) is much cheaper than the equivalent fare from LHR.
4) Personally I actually prefer longer transatlantic flights since there is more opportunity to sleep. The YHZ-LHR flight is probably only around 5 1/2 hours versus ~8 hours YYZ-ZRH. If you can sleep on aircraft, YYZ might be better, if you can't, go for LHR.
5) If you go through LHR, try to book a flight that connects with Air Canada's 777 aircraft rather than the A330 or 763. You will have AVOD in all seats and if you fly J sleeper seats. The 763's may have the AVOD and sleeper seats from LHR or ZRH but only 20% of the fleet has been refurbished, so it is a crap shoot.
6) LHR is a PITA for connections and the taxes for going through LHR will probably make your ticket more expensive than via Canada.
7) If you like flying Swiss, another alternative might be to fly ZRH-YUL-YHZ.