Over or under 25 years? If over, do a full tear-out, if under just go with a second layer. The money is in the tear-out. One layer is expensive, two layers is really expensive. So if you plan on living in the house say another 5-10 years, just go with a second layer.

*Not my opinion, but from a guy I work with who used to be a roofer and comes from a roofing family