I wouldnt do either. I would just site back and wait for the first one to get stock. I have had great service from both buy.com and tigerdirect.com, however I hate having to go thru the hassle of cancelling a pre order and the risk my CC may get charged. I did preorder an E90 from someplace (dont remember who) and they said they wouldnt ship or charge my CC until i replied to a confirmation email. Well, one day an E90 just showed up, about a month or two after I got mine elsewhere. I refused the package and eventually got the CC thing straitened out, but i decided then, no more preorders. As soon as someone has stock, you will know if here first and can then order.
The only advantage I see to a preorder is to lock in a good price.