I definitely prefer the soft/capacitive buttons over the physical ones.
I really like using Android over the current competitors because of the speed and effortlessness it takes me to navigate the OS. Part of that is using SwipePad to access 16 of my most useful apps, quick call my wife, or access my app drawer without needing to press a single button.
The soft buttons require little thought while using the physical button requires me to stop my train of thought and direction of travel to press the button.
Also, the addition of the physical button normally necessitates a redesign in how to access the multitasking and/or Google Now, which I feel just detracts from the overall interface.