Those companies simply put parts together, it depends entirely on which parts you pick as to whether or not it will play FSX/XP10 with high settings.

Which is faster: A Ford, or a Porsche? Difficult question... We need to know exactly which car (or computer set up) you're looking at.

Both seem to be reputable companies for purchasing computers from, but you will have to pick the computer you want and then pick the upgrades that you want applied. Try it out and post the specific hardware details that you've chosen.