I moved out a little over 10 years ago, but I was born and raised and lived 40+ years there, so I do have something of a "home" perspective.
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If I were to split into three states I'd do something like this above. If two, I'd keep the same southerly east-west line. (I do realize I am splitting San Bernadino County.)
The primary reason "for" splitting is that southern and northern are, essentially, two different worlds. Neither one identifies with the other, or really respects the other. The mountains between Los Angeles and the central valley literally divides the state and only fosters the cultural separation.
A benefit to Californians would be expanded representation in the Senate. As it is now, California' Senate representation is watered down. Another benefit to that would be getting Senate representation that is more likely to be actually representative of the constituency, especially if the 3 state option were done, and the rural areas not overwhelmed by the larger population centers. This, of course, would be a big reason why other states would fight a split.
As a general rule, smaller government is better, or at least more responsive, government. In theory, at least. Split into like areas and state government would be better situated to handle the needs and concerns of their areas.
Water, Ah, water. The issue that really gets people worked up. I don't have a link handy, but I have read that northern California would rarely suffer water shortages even in periods of drought if it didn't have to send so much water south. A split would give the north more say in water distribution, though the existing contracts would probably still stand.
There's some thoughts. More positive than negative.