You want as many nominations as possible. Each nomination is actually a SLATE of 10 names. So it's not just that you got a nomination, you got on a nomination LIST. The thing is, they only take 1 name from each list for congressmen/senators and give an appointment. I.e. Your nomination with your congressman is on a list with 9 other people. The academy is only going to give 1 appointment based on that list. The more lists you are on, the more chances you have for an appointment.
If you don't receive an appointment from all the lists that you're on, you still have one more chance. On these nomination lists that are available, with 1 appointment each, (For congressional lists; 100 appointments for the presidential list); only makes up about half of all appointments. (500-600 appointments) Then, all the individuals on these lists who didn't receive one of the appointments for being on the list, are all compiled together into one more giant list. The academy then will choose the remaining 500-600 +/- appointments from the giant list. (National pool).
So get as many nominations as you can. It's like a lottery ticket. The more tickets you have; the more chances you have to win.