The reason forum members talk about the CS (Carbon Speed) frames is because they have been happy with them. Frames from other suppliers have had problems such as blocked internal cable routine, sloppy bottom bracket sleeves, misaligned brake caliper mounts, and rough topcoat finishes. Peter, who owns xmcarbonspeed.com, has mentioned the cheaper frames might be seconds but we really don't know for sure. I've read about people who have purchased frames from Flyxii.com and they have been about 50/50, about half of the frames have minor issues. Peter may hand pick the better frames but we do not know the for a fact, even though his frames seem to have few problems.
Remember, once you get your frame and you find a problem, it can be costly and time consuming to ship the frame back to China. The prices are already very good, so saving another $100 is not worth the possible headaches. You may get lucky and save $100 by ordering a frame from AliExpress, but I think the odds are against you.