I cannot find any supporting documentation explaining why to use QGIS 64 bit instead of 32 bit or vice versa on www.qgis.org.
Is there any available?
I believe 64 bit should allow you use more machine memory than 32 bit, which may be useful for large files/datasets.
As you correctly state, the 64 bit version allows for using more memory than the 32 bit version. One example where this is important is high resolution printing, there are real world examples of crashes with huge printings which are resolved by using the 64 bit version.
Compared to that, the 64 bit version has not really any downsides - apart from the fact that it only works on 64 bit machines. It might have a slightly increased memory usage but I have yet to find a scenario where this would even come close to outweigh its advantages. So while it's technically possible to use a 32 bit QGIS on a 64 bit operating system, there is no advantage in doing so (unless you require a 32 bit only driver like ms access, thanks @Techie_Gus). On the other hand, on a 32 bit operating system, a 64 bit QGIS cannot be started.
If you need to work with Access databases within QGIS, you're bound to work with the 32 bit version as there is no 64 bit ODBC driver for Access. The good thing is that you can have both 32 and 64 bit versions of QGIS running alongside each other on the same machine, provided it's a 64 bit, of course.
If you're trying to export large, high resolution compositions, then prefer using a 64 bit build. Otherwise the memory limitations for 32 bit builds can be restrictive.
