I've had both. I loved my 2000 7.3L, it was bulletproof. I had nearly 300k with no significant repairs. I would still have it if I hadn't totaled it in 2013.
That being said, the 6.7 is better in every way only the jury is still out on long term dependability as I only have 130K on it so far. It is faster, more powerful, quieter and more comfortable than the 7.3L. I don't like the DEF, but I guess that is just the way of the future and I have had to replace the exhaust sensors.
I couldn't buy a new one, the cost is just crazy, but I got a good deal on a 2011 and am very satisfied.
Good luck with your decision.
