Maybe something for the pixelpeeping department, but anyway:
We've probably all asked ourselves that question, where to upsample, in Photoshop or ACR? From Adobe we've always heard that both algorithms are very similar
There's an interesting post on the Adobe ACR forum where Adobe engineer Chris Cox confirms that ACR upsampling creates some artifacts that aren't there in Photoshop upsampling.
Quote from that thread:
"For ACR, Thomas chose a resampling kernel that gets good results but can result in more ringing artifacts than the bicubic family of resampling kernels"
or here on Adobe groupbrowser http://adobe.groupbrowser.com/t155466.html