Welcome to the ORG.
If you want to hot-rod the truck, go with the 6.0L. Alot more options down the road, and better performance vs investments. However, you have to ensure the truck is mechanically sound before you make the investment.
Read the following threads, and don't rush into buying a 6.0L:
Things to consider about owning a 6.0L
Write Up: Bullet-Proofing the 6.0L PSD.
If you go with the 7.3L you have a motor that is going to last you quiet a while, but the hot-rod potential isn't there. Your concern goes from the motor to keeping the 4R100 Trasmission healthy.