I don't own a 6.7, but my observations are below.
The 6.0 is significantly more powerful than the 7.3. The 6.4 is marginally more powerful than the 6.0, and the 6.7 is marginally more powerful than the 6.4.
MPG wise, 7.3 gets better mileage than the 6.0. The 6.0 got better mileage than the 6.4. The 6.7 gets better mileage than the 6.4. The 6.0 and 6.7 are pretty similar in MPG.
Parts/durability wise, the 7.3, 6.4, and 6.7 are more reliable than the 6.0. The 6.4 is more durable than the 6.0 but has more "technology" to go wrong. The 6.7 is also more complicated than the 6.4 in some respects, but mostly better than the 6.4.
Finally, the 6.7 requires DEF.
In summary, the 7.3 is similar to the older Cummins. Less to go wrong, but less right with it. YMMV.
