I saw couple my neighbors installed roof rain gutter. But in southern CA, we rarely rarely rarely have rain. any pros and cons to install roof rain gutter?
It helps keep water away from your foundation. Also helps with any potential erosion, but like you said, not much rain. Sometimes when it rains though it's pretty strong so the runoff from the roof will be pouring down from 30 ft or so and will hit the dirt/plants directly which could damage the plants.
Considering it doesn't rain as much, if 5" is cheaper, go with it.
The purpose of rain gutters isn't really how much water it can hold but how much it can drain off... and if it does fill over, if it's raining that much, does it matter?
check my HOA design guide.
My HOA guideline says "Diverters, downspouts, and gutters shall match those originally installed by the Merchant Builder." and my builder used 2"X3" downspouts.