You said that they are sometimes called the same. They are both lithium but are different chemistries. Try wikipedia to get info on LifePo.
Weight will be similar.
Li-ion 18650 batteries which I have used and built into packs my self have a 4.2v fully charged and a 3.6 nominal voltage.
As I have never used LifePo I have looked into it.
I am now starting to use Li-po, lithium polymer but thee are more volatile than the other two chemistries.
Not sure about the chargers, they are probably different if the voltage in the batteries are different.