Becerra would probably be one of Hillary's best choices, and might even the best choice against Ted Cruz or Marco Rubio.
Julian Castro would be a disaster because of his service in the Obama Administration. Hillary can't afford to pick another Obama cabinet member, especially in a close race (same reason neither Tom Vilsack or Kathleen Sebelius will be on the ticket).
"Independents" are likely going to be turned off by Obama, making his cabinet members politically toxic for a few years. If it just Julian, or just Hillary, it would be a bit different, but both could cause trouble for Democrats.
If Obama is that unpopular in November 2016, Hillary isn't going to win. And I'm pretty sure any Democrat would be tied to the President.
