I remember a couple of years ago the Qantas GM for Switzerland telling me at a do in ZRH that QF had been looking at SIN/ZRH for some time but decided that it was only marginal. I can't recall his reason. Marginally profitable? I've travelled this route for years with SQ, occasionally with LX or QF via FRA, and other than when the SARS scare was around, the planes are full (or fullish).
As for choice on the route, SQ beats LX hands down.