totally depends on what you want to do with the truck. If you want to go fast at the drag strip by dropping a few hundred bucks, definitely go the 6.0
If you work your truck hard and want something that is proven to last forever and be a good tow rig, I'd go with the 7.3.
I'm not gonna dis on the 6.0 because it's a great engine platform, all the add-on emisson crap from FORD kinda crippled it in the beginning, but even now for 05 and 06 I'd say they're a great truck.
It takes A LOT of money to make a 7.3 fast. I mean what you spend to get 14's in a 7.3 you can have high 12's with a 6.0.
Me I think the 6.0 being a smaller engine is being squeezed harder and may have a shorter lifespan, just because it's being worked harded. But this is just conjecture, there will no doubt be many 6.0s that end up with 200, 300 thousand and higher miles. Heck I have an unlce who has one with like 190K on it being worked hard, an 03.
I like the "feel" of the 7.3's power band for towing and a work truck. It makes a ton of power/torque down low. The 6.0 seems to love to rev, but the torqshift does do a good job of keeping it in the powerband when you're out on the road, towing etc.
So if you're really sold on performance and going fast, go 6.0 no doubt. If you want something that you can count on to go 400K-500K and greater if you set it up right, go 7.3 until the 6.0 gets a little more track record under its belt.
Just my .02
