Difference between inotify and epoll
The biggest difference is that epoll can be used for ANY fd. This means it's good for watching all types of ways to communicate data. Sockets, IPC, files, printers.. anything. inotify is for filesystems only.
However, because inotify is specific to filesystems, you can receive notifications on a wide array of filesystem-specific attributes, such as file attributes and the file being read. These things are not possible via epoll.
In fact, inotify returns a file descriptor - which means you can use epoll to determine which inotify FD's you should call read on. So the two go hand in hand to some extent.