Q:

Disable antialiasing inside Xcode?

How does one disable antialiasing inside the code editor in Xcode, but not system-wide?

A:

For Xcode 3.x the necessary terminal command is: 
defaults write com.apple.xcode AppleAntiAliasingThreshold <some integer value>

For Xcode 4 & 5 it is:
defaults write com.apple.dt.Xcode AppleAntiAliasingThreshold <some integer value>

For Xcode 6 you may first need to enter the following at a terminal console:
defaults write com.apple.dt.Xcode NSFontDefaultScreenFontSubstitutionEnabled YES

Then, as per usual:
defaults write com.apple.dt.Xcode AppleAntiAliasingThreshold <some integer value>

Using AppleAntiAliasingThreshold 24 should get you what you want.

