My impression is that raw-conversion and other tools in ACR and LR are essentially the same.
LR is a Workflow solution while ACR on it's own is not really a solution, it's a part of photoshop. The tools in Lightroom are parametric, that is they are "recepies" on how to do an image conversion, with the conversion done when needed.
If you use ACR than you need to save the image as a TIFF, prefererably 16-bits. It's going to be fat. If you want to change a single parameter in ACR you need to redo everything. With LR it's just a touch. LR is also a compete DAM (Digital Asset Management) application. What LR is lacking is pixel level image manipulation.
So my advice would be that everything that can be done in LR should be done in LR. When Photoshop is needed open Photoshop from Lightroom.
Can anyone help me understand where corporate is going? I'm dabbling in LR and learning ACR and wondering why they are both there. I'm having VHSBetadejavu. ACR was the first effort for RAW files so why LR? I'm hearing that the engines in LR [sharpening etc] are better than in ACR. And LR loves caters to package photography. But there is no set of selection tools as in PS. I write software for repair businesses and I know the misery of trying to maintain two "similar" solutions, keeping track of how they are different and why while they are still cousins. But, I digress, would anyone care to suggest that the LR PS ACR triangle is going to integrate/resolve [make up?] and how?