straight from xs
Unless you are in a competition class where you are limited by batteries and burping, then you will probably never notice a difference to be honest with you. The XP series turned out to be stronger than we originally though they would be when we designed them. If you have a battery bank of D3100s and a bank with the same number of XP3000 you probably will not see much of a difference.
Yes, you can run the D or S series under the hood and the XP series in the back with no issues. That is kind of what we had in mind when we designed the new XP Series. Most people complain about the price on the D3100, so we offered a battery (XP3000) that will run neck and neck with it at a $50 lower cost.