The price difference is substantial... is the quality? Anyone have any experience in this? It will be put on two different bikes used for a 1+ year self sustained tour. (one bike for me, one for my gf, i'm not taking two!)
I personally prefer the shifting of the Shimano cassette but have mostly SRAM 9.0, 950, 970, and 990 cassettes distributed across the eight wheelsets for my five bikes and I'm very happy with them. I don't race so losing a few milliseconds to shifting isn't going to amount to anything. SRAM cassettes also have a reputation for having the larger cogs bend so if you're a big powerful pedal masher, you may want to go for the XT or maybe a good old HG70. I'm personally a silky smooth spinner type that only weighs 150 lbs.
I'd downgrade to a lower level. Forget the lightweight stuff if you are going for a year-plus. I've experience with both SRAM and Shimano at the levels you are discussing, but for my purposes, I prefer the strength and durability of the lower levels, such as LX or even Deore. The lower level stuff also is cheaper so when you inevitably come to replace it, your funds won't be drained by the expense of high-level componentry. Having said that, I slightly prefer SRAM over Shimano.
One thing I did notice when I broke the rear dropout of my touring bike and rode on for 400km without realising, is that when the chain derails in between the spokes and XT cassette, it becomes a mighty difficult chore to get the chain out from behind the spider on the roadside with limited tools.
Oh and also with the spiders, you can't mix and match cogs as you could with the other models. It's probably not that important to you unless you want to go the Sheldon Brown/Harris Cyclery route with a better range of cogs.