The latest 7750 is a very good and reliable movement as verified by The Zigmeister. A lot of the parts are interchangable with the genuine Swiss 7750 so service is easy and available.
For the 21j movement, I haven't seen any extensive review of it. As far as I know, there is no service or parts readily available. You can only replace the movement if there is a problem.
The price difference also goes to the saphire crytal comes with the expensive version. The cheaper version only comes with mineral glass.