It seems that DPP has some "magic" that is more effective in suppressing the pattern noise you sometimes get in deep shadows in 5D2 raw files, even at low ISO. Other than that I would rather use ACR but you if you just use the nioise reduction in DPP and nothing else and then save as a TIFF you then can't open in ACR as a raw file. However I wondered if the way round this is to import into lightroom when you would have access to all the adjustments in the Develop module?
I actually have lightroom 3 which I bought a while back because it was on special offer on Amazon however I've not actually used it becauseI find Bridge/Photoshop easier. I would have tested this myself but opening a file in my Bridge folder in LR3 seems beyond my abilities until I can actually spend some time understanding the programme.
As an aside, I wish I hadn't because you can now get LR4 for the the same price, whereas it cost another £60 to upgrade!