I have only been to QQ a few times; therefore, I may not get a complete picture of it. The plus sides of QQ: young girls, FS widely available. Minus: communication. Some speak little English; services vary with the girls and their mood. Facilities are above-average compared with average Asian spas but not spacious as Flirt; SF2 may have limitations.
It's certainly not true that girls in Flirt or Premium are 30+; I would say most are under 30. Servies are not 100% guaranteed in Flirt but usually better than many Asian spas and more standardized. The MAP would go nude in less than a minute when in the room, and the rest depends on how you two click. Before the pandemic, kissing is usually welcome; same for DATY. FS is not supposed to happen, but it does offer by a few girls. Some provide CBJ too. In short, GFE is usually met, and the mannerism of the girls are good. Also, I never heard of communication issues, and all the girls speak English well. The setup is more upscale, and the facilities are more spacious. However, It used to have plenty of drinking water, mouth wash and towels but now no more drinking water/tissue and one bottle of mouth wash/session; towels can sometimes be limited. Price-wise, the charges are similar; therefore, I often go to Flirt and seldom to QQ. I vote with my feet. But, it's also true that QQ has no shortage of supporters. So it's a game of everyone's preference. If you prefer Asian girls, FS and save a few $, QQ may be the choice!
The above are my observations. I hope it helps. Those who often go to QQ can correct any of my assumptions that are not correct.