No body can tell, I do know if you want a popular model preorders wont last long. apple stores will only have a certain amount of product for preorders, and a certain amount of stock for non pre orders, once room for pre orders fill up the timing will change.
Although you wont be charged until your device ships, so you could alway preorder on tuesday, and if its not available for pick up on launch you could try going the non pre order route if you have the time, if you get lucky you get cancel your reservation.