You can't really set it to the USB or iPod to start up with, let me clarify this, you can set it to either of these, but the problem is, while the system is starting up and reading the files it reverts to the WB (if you have this on) because it takes so long for the iPod or USB to start, that it basically decides that it is not available and reverts back to the next media option. You can try turning off the WB and seeing if it will stay with the USB, but it may revert to AM or FM. One other option that may work, is have a very few songs on the stick that you are using and this way it can read them and start up before it decides to time out.
Unless they make changes in a future update to the way the info system boots up, you may not ever be able to get it to start on a heavily loaded iPod or USB device, I usually shut it down with XM playing or FM, this way I can have the WB on and not have my system start up with it playing, since FM or XM will activate before the system decides it's time to go to another option.