I wouldn't recommend the JAL flight. Only one service each way each day - very late afternoon/early night. Check-in opens 3 (or maybe 2) hours before flight with no early check-in available at Singapore or KL.

The Cathay Lounge at KUL is nothing special, but I suppose is ok. Of course the lounges at Singapore are still nice.

For ~1 hour in the air, I'd go with better and more convenient timing from Malaysian or Singapore.