The embroidery module is not optional on the 830, it comes with it, period. The 820 will not come with it and can't be added, but it's also missing some features of the 830.
While the larger hoops size was a definite draw for me, I wanted the sewing features of this machine that my 630 doesn't have (nor did the 730 I traded for it).
1. Dual feed
2. The ability to send any stitch (or combination of stitches) in the machine to the embroidery module
3. Stitch editing on screen (this is beyond cool!) and stitch tapering.
4. The new buttonholes. Fabulous! You can now adjust both the bead of the buttonhole and the cut opening!
5. The "hover" feature of the presser foot. To die for.
Have you "Tinkled" today?