/proc and /sys are usef for different purposes.
/proc/modules does not actually exist until you call it, at which point it is generated. For instance, if you query lsmod.
/sys/module is a directory which contains other directories. These directories contain files which have information about many things, which may include currently running modules as well as, without burdening you with technical jargon, other stuff. It is used by the kernel to figure out what to do.
simon@indigo-prime:~$ cat /proc/modules | grep processor
simon@indigo-prime:~$ ls /sys/module/processor
Here you see that there is information in /sys/module which is not in /proc/modules.
Of course, I can be more specific if I understand what prompted the question.