This is purely a guess. For the past several years, Night of Joy (and Rock the Universe over at Universal) have been the Friday and Saturday after Labor Day. So, next year, it will probably be September 11 & 12.
As far as acts go, the "headliner" tend to alternate between the 2 venues with the exceptions being Mercy Me doing several years in a row at Night of Joy and Switchfoot doing several years in a row at Rock the Universe.
I would not limit yourself to one event or the other. I would wait for the concert lineup to come out then decide. I will say, Universal has offered fabulous deals in advance on their "rock your weekend" tickets. Admission to the parks Friday Saturday and Sunday along with concert tickets for less than the normal price of a 3 day ticket. This year you could get it with or without park to park access.