I tend to puchase most everything from Amazon, I have nothing against InteractCD, have not bought yet, but would depend on the price difference at this point. Free shipping on Amazon! I have both stores in my PC games folder under favorites.
I love Interact too, but being in Canada I have to pay postage per game not per order like you lucky folks in the US. So if I can get the games at Amazon.ca with free shipping, I go there first. I never pre-order and I find both Amazon and Interact have excellent prompt service.