Ive got a 99 7.3 as well. Use it as my DD and to tow/haul when I need. Ive towed car haulers, 26ft trailers, 15,000 lbs dump trailers with no problem, and throw a pop-up camper on it for trips. Its has about 194,xxx miles and wouldn't hesitate to take it anywhere. I do the routine maintenance, and get the tranny serviced every 30,000 miles. Had to replace the water pump about a month ago but no biggie. I am the 2nd owner and put about 70,000 of the miles on it personally.
I have also had many friends own the 6.0 with mixed results, but have personally picked multiple friends up cause their 6.0 was in the shop. Personally I wouldn't get a 6.0 because of all the stories and bad reviews.
Advice: If you get a 7.3, get an extra cam shaft positioning sensor. About $35 at Autozone. They go out on the 7.3's around 150,000 miles and your truck just plain wont start. It will turn until the battery dies but wont start.
Thats my 2 cents. Good Luck! :smiley_drive:
Paul