To tell you the difference. Memfiles does not actually put the entire file in memory, but makes you belive it is. It uses the memory mapping unit that is normally used for ram swapping, but in this context it reads the context of the file as soon as the file is accessed. This needs the size of the file in address space. In your case it would mean it is not possible to use memfiles on a 32 bit system, but on 64bit no problem at all.
FileStreams continuously read the file, therefore you do not have random access to the file.
