Based only on available connection protocols between the older MacBook Pro and a new MacBook Pro with Retina display, AirPort is probably the way to go. Under the AirPort pulldown menu, use the "Create Network..." option on the MacBook Pro with Retina display and then join that network with the older MacBook Pro.
If a Thunderbolt to Ethernet adapter is available, patching via Ethernet would of course be faster than WiFi.
If the older MacBook Pro had Thunderbolt, then that would be the way to go via Target Disk Mode.
While both laptops have USB, Target Disk Mode doesn't work with USB2 or USB3 due to the USB protocol itself. That is, the cables, connectors and communication protocols of USB don't support it. Or, to put is another way, what cable would you use to patch two MacBook Pros together?
Each computer has the host connector (type A) and there's no type A to type A USB cable (at least, not that I've ever seen). I've tried adapting a type B connector back to a type A connector while having a MacBook Air in Target Disk Mode and have not been able to mount the device as it would via FireWire or Thunderbolt.
Of course, Apple is known for taking the existing technology and making it better. It would be great if Apple not only enabled Target Disk Mode via USB, but provided a Target Disk Mode USB cable as well. Now that FireWire seems to be going the way of SCSI (remember the 25 pin SCSI connector on the PowerBooks? Well, most people don't), it would be great to maintain this feature. I've found it to be immensely helpful from quickly moving data from an Apple laptop to a desktop to running disk recovering and disk restore tasks.
At the same time, I'm also accepting that it's a feature that Mac users will just have to learn to do without (like Final Cut Pro 7, extended keyboard keys remapped across the standard keyboard, etc.). Until, that is, we've all upgraded to computers with Thunderbolt.
Other World Computing offers a variety of do it yourself kits for pulling the drive from older MacBook Pro and placing it into an external case. If the drive is a 2.5" SATA drive, the USB adapter from Segate's GoFlex Mobile drives will work with a bare SATA drive. One would probably want to go with a USB option as Thunderbolt is very, very pricy by comaprison.
This thread is a few months old, so I hope you found an easy way to get your data across.
-Warren
