If you already are using pes format, then you will be fine with the 780D. If I am correct, that machine will use the usb stick or plug into your computer either way you wish.
The 780 will use both 4x4 or 5x7 design formats.
I believe the 780 also uses 90 weight bobbin thread. Most other Brother machines use 60 weight. Some of them can be really finicky regarding the bobbin thread.