I rode with a backpack for years then made the switch to panniers. For just just riding to/from work, panniers are my choice because, as so many have mentioned, backpacks make your back sweat like crazy (and you say weather is not an issue, but in SoCal your back is definitely going to sweat!). But for going shopping, or making any stops where you want to leave the bike and carry your stuff with you, the backpack wins. So it really depends on your personal mission. If you do go with a backpack, I highly recommend the Deuter line, with a mesh panel that lets your back breath more.