7.3 is more bullet proof generally speaking, but its going to be more difficult to find one with lower mileage. The maintenance is easier on the 7.3 as well.
6.0 just had some design issues. Some people have had little or no problems while others have spent 4-6k to make them a great running higher performance engine. Head stud replacement, egr delete or upgrade, larger oil cooler are a few things people have done to bullet proof the engine, but again these upgrades/repairs are pricey especially the head studs because the engine has to be pulled to change them. My mechanic drives the 6.0 and he loves it. He also is a magician with these engines, which for him is just parts because he can do everything else. For me not so much. I have 6k in my engine and I am happy with it. I knew what I was going to do before I bought it. Just my 2 cents

 

I did a multistate search to find my Ex, and it has been quite a headache of a vehicle getting it right.

i bought a 05 EB Excursion 4x4 with no rust from a dealer in Il with 91k miles from some retirees that had traded it in for an f250. I doubt it ever saw snow till I got it, but it came with other issues.
1. The autolocking hubs were frozen from lack of use. According to my mechanic if you use them a couple times a year they will last a long time.
2. Cooling system was clogged with sediment, causing high oil temps. Thankfully this has been covered by the seliing dealership as you dont want to know how much the new shortblock, heads, arp studs, front and rear heater cores, radiator, hoses, gaskets, and install added up to.

Now I have what is commonly referred to as a bulletproof 6L, including Arp headstuds to prevent blown head gaskets, EGR cooler delete, new oil cooler, and when combined with a 4" exhaust and a good tune makes for a wonderful tow vehicle for my family of 6.

I am going camping with 3 other diesel excursion families this week and they all have 7.3's. Two of them with replacement trannys and upgraded converters, but the motors are bone stock and running strong well over 150k miles.

If you are going to buy, plugging a scantool in the ob2 port to check the oil and coolant temps when fully warmed up is the best advice I can give when buying a 6L. Those temps should never be more than 15 degrees apart. Mine were 74 degrees apartwith no warning lights....before the rebuild.