yeah, i want to know this too. i havent handled any of the h&m collabs nor the +J so i cant comment on quality.
if i had to guess id say that h&m is out to make a quick buck, turning out collab after collab. whereas uniqlo is working with the designer for a few seasons, honing in on a style. ...dunno?
Maybe add the fact that Margiela isn't with MMM anymore so some people are looking it more as a Diesel X H&M collab. No horse in the race myself.