What city are you starting from?
I would prefer to try one of each. I have always been happy in LH F, but have not flown LX F. I believe the SIN route is a 777, which has their newest F product. I would avoid the rather old F seat if it happens to be a LX A340 (reviews vary). The LH seat is good no matter the plane.
FRA is not a great airport to connect in, but the FCT is great. You can use the FCL if you prefer, but many here like the make the somewhat longer walk to the FCT.