Just on cursory examination, the obvious difference I see is the J-flame seems to just drop everything, so you keep your current springs, they just move down 6" along with all the other components. They've had the kit since 2008 and only have one "example" listed on the website.
Also found this affiliate site: http://6ixthfloor.com/affiliates.html
again the same van. Search google images with camburg van and jflame van... the latter again, all one van on multiple sites.
Full Traction on the other hand is what I have on my Jeep, of course mine is a long arm and totally different application, however they are a big enough name in lifts the obvious problem being they don't have a van designed kit.
I would replace the springs before moving them, and that's what the FT F250 kit does, however while it drops the radius arms it doesn't drop them very much, you get into a geometry issue with longer springs- possibly nothing to worry about. There are also longer drop brackets: http://www.fulltraction.com/detail.php? ... 461&NOTES=
I'd get the 4" kit, 6" springs and the longer drop brackets if I was going with FT. Still the kit is not designed for 6" so you lose the engineered comfort and go more into custom application.
Without rehashing camburg or van designed choices, if the one F-250 kit will fit (as opposed to "can be made to work") then other F-series kits are available, and I'm not sure what is available but I know I see F-series jacked to the sky, so 6" shouldn't be a problem.
So that's all just from the pictures, take it with a grain of salt... and try and find some confirmation about how well the F-250 kit fits. I'm still dubious, because why then wouldn't everybody just be buying F-series kits over Camburg and the like.