Good question. They both have advantages and disadvantages. I think the main disadvantage of plastic is that few people re-use plastic bags --mainly because most plastic bags are designed to be disposed from the outset.

However, petroleum plastics are very durable (else there would not be such disdain for them fouling the enviro), so they could easily be more useful than paper bags. On the other hand, there are bio-sourced plastics (mainly from corn) which are very eco friendly, so I don't know why they'd be banned as well.

I think if SF would make a sensible decision, rather than a political decision, they'd just make people pay the 5 or 10 cents for any store-issued bag. The better option is to use bags made for re-use whether plastic, canvass, or other.