I'll second everything boks said. Here are 10 differences I can find:
1) JJ can carry 2 full-size ATVs, LTT only one (1800 lb payload vs 900)
2) LTT smaller in width, with a much larger sleeping area stock (just two inches from a queen sized sleeping area on each side). For me, this was a huge plus... smaller trail footprint with a larger sleeping area.
3) JJ tent setup very quick. For me, meh. LTT pretty quick also, but not "pop and go" like the JJ
4) JJ has canvas tent from what I understand, LTT has "rubberized" nylon.
5) Both are constructed using the same materials and concepts. Same company makes both (Lifetime purchased JJ). My guess is this was in JJ's product development pipeline before the purchase, and Lifetime decided to brand it under their own name.
6) No brake option on the LTT.
7) LTT is leaf-sprung solid axle, JJ is torsion bar suspended. A plus to the JJ in my opinion.
8) Integrated lights on the JJ... they hang off the sides of the LTT. For what it's worth on the LTT the lights are surrounded by very heavy steel plates.
9) Currently a lot more factory accessories for the JJ, next to none for the LTT. Hopefully, that changes over time.
10) Price... When I got my LTT, it was $3k at Costco. For that price, used JJs were on craigslist in my area, but I went this route anyway. I think the LTT now lists for $3500 at Costco. Primarily, this is because JJ has been around for a while, whereas the LTT looks to be new in 2009. Eventually, the used market for the LTT will place these in an even lower price category.
Like I said, I saw some of these differences as a positive in the direction of the LTT, but you may have differing requirements and needs. I personally wanted a profile that matched the width of my JK. Those with full-size trucks may not care about the 91" width of the JJ.
In fact, having just finished reading the Expo West link in full, I would say the LTT has all of the advantages the author lists in his conclusions (sturdy construction, an amazing amount of tent room), with none of the disadvantages (too wide).