Depends on the forecast.
If I don't need many layers (shorts, t shirt, arm warmers, windshirt/maybe windpants, beanie, gloves will easily take me into the high 30s), I'll carry my Nathan HPL20 with a 2 liter bladder (no hose) and one 24oz. handheld. The storage space in the Nathan is limited, especially with a 2L bladder, but a small stuffsack of light clothing can easily be cinched to the shock cord on the outside and rides secure. I know I can fit a Houdini, arm warmers, Dynamo windpants, light beanie, and light gloves this way.
If I need more clothing/layers (rain shell and/or any further insulation), I'll carry my Black Diamond BB with a 2 liter and one 24oz handheld. I love this pack, it rides really well and can pack an amazing amount of stuff for its size.