Recently completed a stay in the Koestler Suite using an SUA and would return. The suite is definitely on the smaller side, but designed in a way not to feel uncomfortable. Welcome gift was a jar of shortbread cookies. Breakfast was very good in the restaurant with prompt service and small but high-quality buffet including pasteis de nata. Also had dinner in the restaurant one night, and was pleasantly surprised by a 20% discount for guests (not mentioned at check-in or when I made the reservation). Front desk is okay, attentive but not necessarily friendly. The only disappointment was the mattress, which felt thin and transmitted motion every time my SO moved, not ideal for light sleepers.

Overall, I would pick GSY over the Churchill for future stays, even though I prefer the Churchill's location. Though for tourists and first-time visitors, GSY's location is great and close to many of the attractions in Westminster.