As an owner of a 2002 7.3 Diesel Excursion from May 2002 until September 2014 and a current owner of a 2017 3.5 Expedition, I can speak to both vehicles extensively.
There is no doubt that the 7.3 Diesel Excursion is a much better long haul tow vehicle, both from a tow capability as well as overall capacity. In addition to being my daily driver from 2002 - 2007, it was our race car hauler during all of our ownership, pulling trailer, car, hundreds of pounds of tools and parts along with 5 people all over the east coast and mid west.
The 7.3 is essentially an indestructible engine with its only downside being the cam position sensor - easily rectified. If I could find a rust free, super low mile (less than 100K) 2002 2003 7.3, I would buy it tomorrow.
Now the other side of the coin. The last year you could get a 7.3 was 2003 (20 years) and the follow-on 6.0 diesel engine is absolute crap (major class action lawsuit against Ford and IH) so finding a good low mile, rust free example will be difficult. Also, the 7.3 engine is LOUD, not necessarily from inside the truck, but def outside, where you could not have a conversation near the truck while idling!
Our 2017 Expedition is a good tow vehicle as long as you are not towing / carrying a lot of weight. My current high tow weight is about 6300lb and the Expy handles it very well, but I'm not hauling 5 people plus 1,000lbs of tools and parts in addition to the car and trailer.
The 2017 has better non towing, around town and on the road manners, is a very comfortable truck and easily hauls lots of people and gear for long distances.
