Practically speaking, I wouldn't change the defaults. This means that if you're using any DLLs (such as plugins) that you don't also have a 64-bit version of, you won't be able to use those plugins (or whatevers). That is, a 32-bit process can't load 64-bit DLLs, and vice versa.

A process can only load libraries (DLLs) that are the same bitness as the process itself.Hopefully this doesn't matter, but sometimes security bugs happen and in that case a 64-bit process may be just a little more resilient. 64-bit processes are slightly more secure than 32-bit processes, both because address space layout randomization (ASLR) can use more entropy and because integer overflows on pointers are a lot easier when pointers are 32 bits long than when they're 64 bits long.In practice this shouldn't really matter - WMP is smart enough to buffer partial reads of files, rather than trying to hold the entire file in its memory at the same time, so it mostly doesn't matter how big the file it's opening is - but in theory on a computer with lots of RAM 64-bit processes can buffer more of the file, reducing I/O operations (especially if you seek around a lot). 32-bit has a maximum user-mode memory address space of 2GB, while 64-bit has an effectively-unlimited maximum memory (2^63 bytes, far more than the physical memory of any existent computer).In the case of WMP, the ones you care about are probably as follows: The differences are pretty much just the same as between any other program's 32-bit and 64-bit versions.

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