That depends on the exact hardware that is configured. So it can change every day depending on what they offer.

And it could well be that a custom built mix (say, the fastest motherboard offered by Alienware and the fastest graphic card offered by Asus) is even a few procent faster.
And it might be that it depends on the game. I mean: game A is faster on the best Alienware and game B is faster on the best Asus.
And it might depend on some random choice of hardware. I mean, overclocking isn't guaranteed, but there can be a subtle difference between two processors of exactly the same model of CPU so the one you bought might be stable up to 10% overclocking, while the one your friend bought is stable up to 12% overclocking.

Why not buy both (money is no object) and start benchmarking with your favorite games?