Well the way I see it is for you to get speed and reliability you will have to spend 2-5k on each engine on top of purchase price( unless you find an already bulletpfroof 6.0). You will need that money to make the 6.0 as reliable as the 7.3 and that much money to make the 7.3 as fast as the 6.0. Some 6.0s get beat on everyday and never have an issue but some don't. My choice would be get an 05+ 6.0, bulletproof it, and get some good custom tunes and you will have a great looking powerful truck.
Only thing you need for a reliable 6.0L is a Monitor. I can't stress that enough. So many of the 6.0L's weaknesses have a symptom, and an OEM or Aftermarket solution that can be addressed as the need arises.
I agree the '05+ trucks are great, you want to take that one more step, find one built after Jan'06 when the last factory updates where made.
It takes way too much investment in hardware to get a 7.3L to put down 400HP+, with a transmission that will support, to compare the costs between upgrading the two. All the 6.0L needs is custom tuning to make that level of power, and the transmission can run with it all day.
1k dollars to bulletproof your truck if you do the work yourself. I love my 6.0 and I wouldn't trade it for anything
Depends on how indepth you take the endevor. I figure around $2,200 in parts, if you don't require heads, and you don't install an external oil cooler. That includes the EGR cooler solution, new oil cooler, and studding the truck - taking the heads off.