There's a setup script that will get RetroPie installed in Raspbian, so that you could start emulationstation (the game selection GUI) manually. The RetroPie SD image has a few extra features configured (http://blog.petrockblock.com/retropie/#retroimage) that I don't think you get when using the script.

you could use something like berryboot [0] and install both on the same SD card.

0 - http://www.berryterminal.com/doku.php/berryboot