Go for Autolite XP104 - there are the best for the 5.0L engine. Motorcraft ones are made by Autolite too, but they are cheaper versions...
Confession: With all the ranting you do about how great these plugs are, I found a steal of a deal on eBay and decided to give them a go. I installed them two weeks ago. They replaced Motorcraft Double Plats (recommended spec) that were about a year old and were still at the 0.054 gap that they went into the engine in last summer.
Conclusion: I didn't notice one iota of a difference between the XP104 and a one year old set of Motocraft Double Plats (or whatever they call the standard spec Motorcrafts these days). In fact, I was just thinking today that my truck ran better with the Motorcrafts as I've had some starting issues lately (could be the winter gas which has been known to happen in my truck). I don't feel like swapping out plugs again because with the luck I've had lately (long story), something bad was bound to happen. I`ll leave the XP104s in there and use the Motocrafts as backups in case I need them.
Recommendation: As we all say, go with Autolite OR Motorcraft, but I think for the money, Motorcraft Double Plats (standard spec) are the best plugs out there, at least in the 5.0. I've also used Autolite Coppers last summer, but didn't give them a fair shake because I had other issues that I thought were plug related but turned out to be a header gasket issue. I pulled the plugs I bought in Kansas after a few thousand miles and they look as good as new.
I did a lot of reading up on these XP104 plugs and there was plenty of psychological improvements mentioned out there on the www. Keep in mind that people are usually replacing worn (and old) plugs so of course there will be a difference. I noticed nothing after swapping out one year old Motorcrafts.
The only difference I see between them is that the Autolite use some Iridium enhancement (not full Iridium like the NGK). All this translates into is a little bit more life. Iridium does not translate into performance benefit, just longevity. Plugs are cheap in the grand scheme of things and easy to change in the 5.0 so I doubt I'll keep them to the end of their useful life anyhow.
I definitely don`t see the Motocrafts as a step down, but equal to the XP104. Go for what is cheapest, or better yet, stick with the standard Double Plat equivalent in the Autolite line if they`re cheaper than Motorcraft.
Whomever mentioned that Autolite are made by Fram, get a clue. Fram, Autolite and Motorcraft are made by Honeywell. Autolite and Motorcraft plugs are probably made in the same facility with the latter being made to Ford spec. That`s not a bad thing.
