Breaking it down:
Rims: The 719 is a much more expensive rim than the 319. It is a welded rim whereas the 319 is pinned, and uses a more expensive proprietary alloy. The 719, according to Mavic, is also about 30g lighter. Deep in the Mavic tech specs (I cannot recall how I found this information or I would provide a link), Mavic claims a slightly higher load rating for the 719 vs. the 319. All of this said, though, the 319 is a really nice rim, especially for the money. It is very beefy, has double eyelets like the 719, looks great and is very nicely finished. I would use it for loaded touring without hesitation.
Hubs: I believe that the LX hubs, or at least the current generation, are non-disc only (US market LX). They are aimed at the touring market. Current production XT hubs are aimed at the MTB market, and all are now disc-brake hubs. It is possible that the hubs around which these wheels are being built are older XT or even European XT, in which case they might be non-disc. Qualitatively, the hubs are very similar, although I am not sure that the LX hubs use the same hardened-race treatment as do the XT hubs. The quality of the seals is identical, I believe. The biggest difference is in the axle (and this might be the source of the claim that the LX is a better touring hub). The LX uses a steel axle, while the XT uses an aluminum axle of a slightly larger diameter with more and smaller ball bearings. The exception to this is the XT M756 series, which still use a steel axle. What is most important for you is that you get the hub which works with your braking system, whether it is disc ISO 6 bolt, disc Shimano Centerlock, or non-disc (US market LX), and that for some reason the wheels are not built with thru-axle hubs which will (I'm guessing) not work with your touring frame. It would be odd to build a touring rim to a thru-axle hub, so I am going to assume that this is not an issue.
You haven't mentioned spokes or nipples, and the preference here would be for double-butted (or triple-butted, but they are rare) spokes as opposed to straight gauge spokes, and brass nipples rather than alloy. It is very unlikely that the nipples would be alloy, as they are usually 3x the cost of brass nipples. They weigh 1/3 of what a brass nipple weighs, and despite Sapim claiming that their alloy nipples are as durable as their brass nipples, the standard in building touring wheels, where weight savings is not a high priority, is to use brass nipples.
More important than any of this, though, is the assembly quality of the wheels. I would much rather have decent, mid-level components that were assembled into a wheel by a reputable builder than top-of-the-line components slapped together by someone not so obsessed with the nuances of building a durable wheel.
My bottom line would be this: if you are looking to save a bit of money, and they will work with your braking system and frame, assuming the same build quality, do the LX/319 combo. And just to confuse you more… it is also possible to work with a custom builder who will suggest and assemble for you any combination of components you might desire.
