Received my J1B in white! Comparing to my J28 1.2:
Both are size small but J28 is quite a bit more generous, especially across the back and in the sleeves.
Both will accommodate a J68 as a liner, though the J1B is more snug
J1B hood sits probably an inch and a half ahead of the J28 hood, so the J1B provides much better coverage
The J1B collar when zipped accommodates the J68 collar, also zipped, without being constricting
The stand collar of the J28 doesn't stay nearly as neat as the J1B stand collar when unzipped
I'm a little torn on losing the front pocket array of the J28. They definitely distorted the profile of the jacket, but it was super easy to access the main pockets + mezzanine pockets. OTOH, the J1B is much cleaner and I would say has similar storage space to the J28, but the pockets are more finicky to access since they're WR zips and on the sleeves rather than velcro and on the chest.
All in all, I really like the J1B and will probably let go of the J28. @jeddyhsu what do you use the detached pockets for? Right now I've been thinking masks but unsure what others have found them best for.