Assuming you are talking about Inferno here. Depends on how much Magic Find you have and how much faster you are with killing in Act 2 versus Act 3. You need to account for a 100% (4% to 8% difference between Act 2 and Act 3 for iLvl63 items) for drop rates. So some combination of killing speed and/or MF is needed. Can you kill twice as fast or wear at least 100% MF? That is your break even. If it is a close call, go with Act 2 as it will be safer and the drop rates for iLvl 61 and 62 items aren't that far apart.
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