An article in today's Contra Costa Times says a 5,000 cfs canal might make more sense than a 15,000 cfs peripheral canal.
In simple terms it comes down to the purpose of the PC. Is it a) insurance against a massive levee failure, most likely an earthquake, or b) creating the infrastructure to increase water exports. If it's insurance, the little canal most likely makes sense if it costs less - and I have no idea of the cost difference.
My favorite quote from the article is from Laura King Moon of the State Water Contractors. She has been pushing the large version for years, and states in this article that 15,000 cfs is what we need. Then she says...
"They presume to know the right answer. It's not possible to know the right answer right now," King Moon said.