More :
it's not because your processor and your OS is 64 bits that you need to compile in 64 bits. Many many application running on Windows 64 are still 32 bits applications : Windows supports this.
Codeblocks itself, as distributed for Windows, is a 32 bits application and it works perfectly, as expected, in 64 bits (OS and processor).
TDM 64 bits version of compilers have standard name like x86_64-w64-mingw32-gcc.exe, so it contains mingw32, I think because it can produce 64 bits application by defalut and 32 bits too (-m32 option).
You may need a 64 bits application for speed reasons (may be !), or for using very big arrays in your code.
But as told previously, this is not totally CB related : it's more a problem like which compiler to use for your needs, and CB supports many different compilers (32 and/or 64 bits on Windows).
gd_on