While my J class flying experience is much more limited than most on this board, I've actually flown both LX and TK J twice - including the TK combination you're considering (ZRH-IST-YYZ), although I booked it with an overnight in Istanbul.

If lounge access is of prime importance, go TK - it's really is spectacular, and huge, to the point that the 2:40 connection is probably a bit short to fully appreciate it. The big negative is that 2-3-2 configuration on their 777s - but in terms of the actual seats I much prefer them to the LX J seats (and yes, they are fully flat), and the food on board is wonderful.