Probably hard for many people to comment (from experience) on that wide a range of packs, but a couple of comments:

"The Gossamer Gear mariposa + uses the most fragile fabric of all."

Well, the "plus" model has stronger fabric than the straight Mariposa. I carried a Mariposa Plus on my PCT thru-hike in 2008 and the fabric wasn't a problem, no rips or anything apart from the external mesh (which seems to get torn up on any pack). I did have some minor issue with the pack strap attachments, but I was also carrying beyond the claimed weight limit in the Sierras, and it didn't fail.
Note that the new Mariposa Plus version has curved stays that are supposed to "greatly improve the fit and comfort of the pack" and "increases the load transfer capability and allows the pack to hug your back better providing a snug fit and virtually eliminating the need for load lifter straps"

Since this is new in the 2009 model, you won't get any feedback from folks on how it works, but I would give Glen of GG some benefit of the doubt here ... it's already a good pack, and as good if not better with this change.

I'm not, however, a super fan of the Mariposa waist belt. It works, it's not bad or painful or anything. I just recall that overall my ULA pack is a bit more comfortable overall (waist belt, shoulder straps, etc). But --- an older model ULA pack --- it's also something like 2 pounds heavier. I have no idea whether a ULA pack of equivalent weight would be as comfortable (and pack comfort is somewhat subjective). I did run in to a lot of ULA packs on the PCT this year, and all of their owners were happy with them, though I would say that was true of other pack types too ...

Since you said you're doing long distance hikes, inevitably an ultralight pack isn't going to last forever --- they can hold up well (I'll use the GG pack from the thru-hike again this year), but they're not going to last for decades of hard use. This for me might change the cost trade off. I.e., if you can get a Mariposa Plus for half the cost of an MLD or SMD pack, I would be inclined to go that route.