From what I've read, it's not unusual for Sundays to be as crowded or more crowded than Saturdays anymore, perhaps due in part to blockout dates (and some people work or have other activities Saturday but not Sunday, while most churchgoers can usually go to DL on Sunday afternoon if they want). It looks like this whole weekend is blocked for Southern California AP-holders, so the blockouts may not be as big of an effect.
Actual rain will keep many locals from attending (and close a very few attractions, and make others uncomfortable). Threats of rain might not reduce crowds, and if the rain clears up the locals will likely come on over. Also, do you WANT to go in the rain? Lines are usually shorter but if it's cold, it's not the most fun way to go.
If you're going to stay 'til park close, Sunday nights are usually somewhat lighter than Friday or Saturday nights--but this weekend a lot of people have off Monday for Veterans Day, so that probably won't apply either.
Anyway, if rain is predicted, my best guess would be that Saturday would be less busy than Sunday, but it's just a guess. If not for the rain, I'm not sure it would make much difference this particular weekend.