I get the impression that GC North is less rammed than GC South. This may be because the North section of T5 doesn't really have what cardholders perceive as the premium check-in area (for what it's worth at T5).

Takeoffs and landings are swapped each day 1500 from 9L/27R to 9R/27L, so it depends on what you want to see. You can generally tell what's going on once you are at the forecourt of T5. If the wind is from the east, as it is much of the time, takeoffs and landings will be towards the west, so you probably want to choose the lounge that gives you the view of takeoff (and since T5 is at the western edge of LHR, you'll get quite a good view). The landings will be a bit boring, as they'll have touched down and turned off long before you get to see anything interesting.

If the wind is blowing from the east, you'll want to be in the North lounge, since there are no takeoffs on 9L, meaning that they will be landing aircraft there, which is more interesting to see from the far western edge, than the early take-off roll you will see at 9R from the South lounges.