Of the two lounges in the main terminal I would go for the South, bigger and less busy.
Somehow I think people just don't want to trek all the way across the terminal from one end to the other and up the escalators. North lounge is the first you'll come across, so after a trek from the CW checkin to the North Lounge is already one length of the terminal. To get to the south lounge is a trek back down the terminal. Good to get some exercise!
That said as you're flying LHR-SIN on a 380 it'll be B or C gate, therefore I would go straight to the lounge in B gate. Much, much quieter.