Benchmarked strictly across TripTiming's 5 empirical pillars: Crowds, Weather, Vibes, Prices, and Safety.
London emerges as the optimal September destination with an overall TripTiming Index of 65/100 vs 62/100 for New York City (+3 point advantage). London experiences more manageable visitor density and shorter landmark queues (carrying capacity score 53/100 vs 35/100 in New York City). Urban navigability is also slightly smoother and more predictable. Travelers also benefit from lower accommodation costs (~$333/night vs ~$406/night in New York City, saving ~$511 over a week). While New York City retains an edge in Weather Comfort (98/100 vs 87/100), London's commanding performance in Space & Crowds secures the overall September recommendation.