I think Eric summed it up pretty well. If you're not aware, equivalent versions of ACR and LR use exactly the same processing engine, although the controls are a bit different. Photoshop itself is an entirely different way of working. Instead of Levels in LR you have the Basic panel - Exposure, Highlights, Shadows, White point, Black point. The Tone Curve in LR works in two different ways, either as a point curve as in PS, but also with sliders for the different regions. In Point Curve mode you can also pick and R, G or B channel to adjust independently. I use LR exclusively, except for things it can't do, like stitching, stacking/HDR, layers/composites, content aware fill...
Mike.