I got a pair of J Crew selvage last fall when they were $50 with an additional 20% off or something like that. From my experiences the quality of the J Crew denim is much better than those from gap. Idk what weight the J Crew denim is, but I think that it's on par with most of the "normal" sugarcane or samurai denims. The gap denim is much thinner, more like the STF levis. The J Crew jeans also had an actual leather patch, hidden rivets, donut buttons, reinforced back pockets, etc. The Gap jeans does not have any of these.
The benefit of buying the Gap jeans is that they are not predistressed, and will fit slimmer. I don't know how well they fade though, I am wearing a pair right now and they seem to fade to a light blue like STF 501s (not great). Also, because the denim is lighter weight it stretches more. Normally I would wear a 34-35 in most jeans but in the gap jeansI could button a 31, and those stretched to a comfortable, slimmer fit. The J Crew straight legs on the other hand, fit boxy and are meant to be worn anti-fit like repro 501s.