I'd imagine the 768GB SSD drive
The SSD would be where you install all of the apps, which would run quite nice. And at 768GB, I don't see any way you're filling that up with JUST apps.
Then put all of your movies (that you WATCH) and your music on the USB3 drive.
The only caveat would be: if you're doing a lot of video EDITING or CREATION then you'd probably want your source files for that video on the SSD drive until you were done.
That's what I do with my PC: apps (for speed) on the SSD, everything else on seperate HDD. Since once the iTunes-purchased movie is on the drive speed is no longer an issue (unless it's really really slow HDD)