Hi Andrew,
I cannot speak for the Quigley, but do have the Sportsmobile 4x4 conversion on our van and have some details on my website of what has worked, what hasn't, what has broken, what hasn't, etc...(
http://www.badgertrek.com/sportsmobile/4x4.html)
Overall I'm fairly happy with the 4wd conversion that SMB offers. It has taken us into some place I would never have expected to see a delivery van a few years ago, and back out again.
Benefits over the Quigley that led us to choose:
-Significantly tighter turning radius
-Lower ratio 4-Low (which is nice when you are carying your household with you)
-I liked the construction of the gear driven Atlas T-Case better than the chain driven T-Case I saw on a Quigley (no clue if its the same one they use now).
-Detachable anti-sway bar (though we just leave ours detached almost all the time now).
-I believe the SMB conversion moves the front axle about 1" further forward, allowing larger tires before fender trimming hits the doors (we are running 37" with only cosmetic trimming, nothing structural, and preserved turning radius).
We have had a few issues with parts in the front suspension breaking (U-Bolt plates, track-bar), but SMB has been very responsive in getting us replacements, and the U-Bolt plates have been somewhat altered to address one of the issues we had.
The spacers on the rear wheels were included, which match the front track-width, but I don't know how much I like them. They presumably make the van more stable, and are nice for the aestetics, however they lead to more sidewall scrubbing on tight trails. I have not tried removing them yet. The full-float rear axle conversion option is a few $k more, and is wider, eliminating the spacers (and also the option to narrow rear track-width if you so wish).
Rear lift is acomplished by some fairly large lift-blocks, which is not on my list of happiest things, but I understand why they go that route, and it hasn't caused us any noticable problems.
We did get 1 add-a-leaf per side for the rear springs as they were sagging a fair bit under the load we carry.
Hope this helps somewhat,
-Erik