I actually don't think North versus South makes a huge difference, HAL move staff over if there is an obvious workload imbalance. Personally I would go North just because it's all laid out more straightforwardly from HEX / Elizabeth / Tube. Time of day / day of week makes a bit of a difference but it's easy to overthink this. South can be somewhat congested between security and Boots, but there is a bit more space between North and the escalators by the BA North lounge.