Doesn't much matter if it's just towing average stuff. But don't kid yourself, a 7.3 has nowhere near the power of newer trucks. The older 5.9's didn't have much power by today's standards either. Towing heavy the new trucks pull a whole bunch harder. The 7.3's and older 5.9's are good engines but agree or not, as daily drivers they were also slow pigs and noisy clattery things.
But diesels aren't only good for towing. It's also about enjoying what you drive. At altitude or on mountain grades nothing comes close to a diesel. I rarely tow and wouldn't trade mine for any gas truck. No more noisy clank and clatter either. The newer trucks are so smooth and quiet and torquey and accelerate so well, that they turn a diesel into something really enjoyable to drive. It's also easy to get more power and make them even faster/more fun, it doesn't take that much. These things will be faster than a gas truck with better mileage, obviously tow better, and be great for everyday driving.
The 6.0..... A good friend works at a large diesel repair shop. I once asked him what goes wrong with the 6.0's. He said, "Better to say what doesn't go wrong. Those things are awful." His brother is a diesel mechanic who had a much modified/built 7.3, a real good one......it towed about the same as his current early DMax which is completely stock, running only mild tow tune. There's a difference.
