While I was a ford technician, I had the opportunity to experience many high mileage 5.0's and 4.6's. The honest truth is, they are both good engines, but I've never seen any 4.6L last as long as a 5.0. 4.6L modular motors suffer from timing chain tensioner wear, which is normally the first thing to trouble the motor if it is maintained correctly. They are a teflon plastic composite that slides against your cam chain. The hardest thing on these timing tensioners is mileage itself, that and idling. I use to replace motors in police cars once a month on average because they were idled to death. The excessive wear caused by the idling on the cam chain tensioners caused them to get eaten up to the metal, shavings in the pan, chains would break, that kind of thing. Replacing the tensioner is not an easy task for someone who is inexperienced, even experienced enthusiasts will lament it. All the accessories have to come over to get the timing chain cover off. High mileage 5.0's suffer entirely from valve seals, low compression from rings, and occasionally if it is beat on, you get twizzlered oil pump shafts (easy to upgrade). 5.0's do not have an all around problem to be delt with. They are like most any other small boock V8, you will replace a water pump every 120k miles on average, occasionally you will do a lower intake (rare though) but in the just 5.0's tend to last longer. 250k 5.0's spray 150 shots all day long just fine, they might blow a little smoke but its nothing to worry about. I did a H/C/I blower swap on my vert with 120k miles on it, and drove the motor all the way till when I took it apart, 50k later, worked flawlessly. You will get biases here and there, I'm sharing my experience working on them.
Let's give the 4.6L's more time to rack up miles before we talk about 5.0L's having more miles on them than average. Some of those 5.0L's have 10-15 more driving years on them...and parts replaced...that help them acheive that.
I think when it comes down to it, they will end up being pretty even in mileage...
I first posted in this thread on 08-18-04 with 166k miles on the ticker. I now have 192k miles and I still have strong oil pressure and it still gets great gas mileage.
Im not saying that a 5.0 is better than a 4.6, im just giving an idea of what a 17 year old engine can do. (its out of an 89 that was wrecked)