Differences between compiling for i386 vs x86_64 in Xcode?

Question

What are the differences between compiling a Mac app in Xcode with the Active Architecture set to i386 vs x86_64 (chosen in the drop down at the top left of the main window)? In

Answer 1

Unless you have a reason to compile for x86_64, I recommend just compiling for i386 (and PPC if you support that). Read Apple's stance on the matter:

Although 64-bit executables make it easier for you to manage large data sets (compared to memory mapping of large files in a 32-bit application), the use of 64-bit executables may raise other issues. Therefore you should transition your software to a 64-bit executable format only when the 64-bit environment offers a compelling advantage for your specific purposes.