Thank you. For those who come across this post, rather than have you post jumping across the answer Hub, I thought I’d paste in the main answer given by Ben Marsh:
The editor requires a lot more memory than standalone games do, but not everyone has a 64-bit version of Windows.We make the assumption that you’ll want to target as many people as possible with your finished game, and that most games can run in 2gb of memory. So by default we use 32-bit executables.
You can absolutely build a 64-bit-only game if you need to… It’s likely that the 32-bit version will still be faster if you can fit in memory though: most general purpose code won’t be any faster using 64-bit registers, and the larger pointer sizes will just result in more cache misses.
So if I understand him correctly, 32 bit is typcially better as you reach a wider audience (not sure how true that is in 2017) and with ferwer cache misses. The reason you may want to use 64bit is due to a memory limitation with windows 32 bit. He mentions 2gb of memory. Is this system ram? That seems incredibly low considering most games require 8gb for their min spec. Also, could we get some info on how to test to see if your game is within this 2gb memory limit?