"nice page sliding animation" vs "[losing] all of your data" does not sound like a difficult decision to me.

Contrary to Chrome which pops the history stack instantly, Safari's "nice page sliding animation" is completely reversable if you don't release the gesture and "put things back in place". The animation is not a gimmick, it has a very real usability purpose.

actually, there's an animation on the side of the page in chrome that's reversible as well

To complicate things even further: only for 2 finger swipe, not three. Gestures in Lion are a sad affair.

there's an animation on the side of the page in chrome that's reversible as well