OP, buy it on Thursday if it's the only day you can, Leopard is enough for you for first days/weeks. Also, if it's your first Mac, you may get disappointed because SL has some bugs of course
Why don't you order the MacBook from the Apple Store on-line on Friday? I assume you will get the SL disks in the box, or at least be able to then get the upgrade for free. (can someone confirm this?)