According to "Leerboek wentellagers", SKF, 1985:
"In general, maximum load on a ball bearing is proportional to outer diameter of bearing times width of bearing (where width is measured in direction of axle)."

The outer diameter is determined by the hub shell and basically the same, right? So LX has an advantage here. I don't count my own positive experiences with tools as representative for the general quality of a product, but according to many bikeshop guys, who also happen to cycle around the world from time to time, (on the wereldfietser.nl forums, including people who build and sell touring frames/bikes for a living) say external BBs often last only about 10000km, square taper ones often last 50000km. Not my mini experience, but their expert experience. Most of the time they advise LX hubs and a BB-un54 in combination with Sugino XD cranks. Deore dérailleurs will get you around the world too. When you're not racing, there's no point in paying lots of money to get the latest ultra light race tech although it can of course feel very nice to be riding with extra bling

I just checked some recent Shimano techdocs and apparantly there are XTR rear hub models with smaller bearings (3/16") and other models with bearings the same size as LX (1/4"), so this is making a general comparison LX vs XTR more difficult... Same with XT apparently. (higher model numbers, smaller bearings)

Using mountainbiking as a test isn't appropriate for touring bikes imo, the use is very different. Distances on an MTB are much shorter, touring bikes are constantly heavily loaded while MTBs get more short hard shocks and cleaning/maintenance of an MTB is usually much more frequent. Replacing parts on a mountainbike when they're worn out is also less crucial at home than when you're touring through Mongolia or the amazon rainforest.

In the end, I'm sure you'll be happy with either hub. The question is how far you want to optimise your ride for touring use and how much money you really want to spend. And of course, if you're only touring in the US or Europe you can probably get your bike fixed in any of the many big cities, making mileage before worn less crucial.