"In essence" is the better choice. I would not suggest using the first sentence by itself. It would be okay to use it to contrast with "outside" differences: "On the outside, we may appear different, but on the inside, we are all alike."
October 7, 2012
"on the inside" is very abstract, and could mean a number of things. The correct phrase is "in essence".
October 6, 2012
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