given the long lives Ive gotten out of deore and xt stuff, I would imagine that the diff between 105 and ultegra is probably a weight thing? ie that the tolerances and cone cup material quality is right up there with the 105 still?
I have noticed that xt stuff used to be of higher quality compared to deore, and perhaps better seals like fiets said.
for touring purposes, you may want to try to get informed answers about ultegra , ie if the "lighter this and that for racing to save x grams" thing would be a negative?
going from my experience working and servicing hubs that range from low end, to middle (deore lx) to XT stuff, I have noticed that its worth it in the long run for XT, as they seem to seal out stuff better, and therefore grease stays cleaner--and seem to have slightly higher quality cone metal and all that, and higher tolerances of better machinng etc.
personally Id value better sealing over saving a few grams, and to my unexperienced view, ultegra may be just a lot more expensive vs the same thing with 105 for a scant grams heavier
to me, both are clearly great quality, so maybe try to find out if the sealing is better on the 105s, or if the same, if it comes down to twice the price for ultegra because you save 50 grams or something.
its the old "cost/value/functionality" balance thing