That 2nd dial on the a100 is what makes the interface so annoying. On the XTi all the buttons and dials you need to adjust stuff while shooting are on the right side of the camera, leaving your left hand free to keep the camera ready to shoot.
For example, to change the ISO on my XTi I hit up, and then up or down to the ISO setting I want, and then half press the shutter. All that is done with my thumb and index finger on my right hand. Barely changing my grip, I don't even need to lower the camera. On the a100 I have to turn the left dial to ISO (good luck doing that without looking) with my left hand, press the little button on the top of the dial (again, left hand), then with my right hand use the up/down buttons to choose the ISO, and then half press the shutter.
The a100 does feel better in your hand, but once you realize you should be supporting the weight of the camera with your left hand its pretty much a non issue.