Here's a direct comparison:
http://www.brother-usa.com/HomeSewin...E#.VTk2s5Ocj4Q
My BFF has the Designio and loves it. It's a great machine. The Designio is actually just a PE770 in a different dress and comes with a bunch of supplies that the 770 does not. It's easy to use right out of the box, although if you watch all the Utube videos while you're waiting for it to arrive you will be more informed and ready to begin. That's what my BFF did and even though she had never used an embroidery machine before she did great her first time stitching a project.
The SB7900E has one big drawback that I can see in the comparison- it doesn't have a USB port for importing designs. You have to use design cards. That's probably why it comes with software. Personally I'd prefer a USB over just a card slot. My Janome machines have both.
Cari
Edit-The Designio can import from a USB drive, no need to connect to a computer.