It will depend on which runway is in use for landings and which is in use for takeoffs. I tend to head for the South lounge as generally the aircraft taxi and take-off in an easterly direction from the south runway (at least in my last few visits). The north lounge gives you good views of the aircraft approaching the north runway for landings. So if this is the case then it will depend what you prefer to watch.

Both offer decent views though.