The USMC does, in fact, fly fighters.  As far as I know, the only relationship between the Navy and the Marine pilots is that you can become an officer in the Marines through the United States Naval Academy.  You can also do this through other programs such as PLC and OCS.  The Marines, however, also fly helicopters, so you might be assigned to one of those.  As far as difficulty, I have heard that the Marines don't care as much about your grades when it comes to pilot training, but study hard anyways because you should do your very best  After all, if one guy studied harder than you, he'll probably get the airplane.  If you really study hard and you keep in shape, then you should have a good chance of flying a fighter in the USMC.