What if a knight, charging on horseback, uses a longspear? Clearly, they get 1.5x Str bonus and 2x power attack. But they don't get double damage, and they can't use the longspear one-handed. Even at 1x Str and 1x Power Attack, the lance will normally do more damage. Only at a -5 Power Attack on a Spirited Charge does the longspear do more damage than the 1x lance. On the other hand, Deadly Charge and Unstoppable Charge really throw off this calculation and make the longspear much better (if we use 1x Str for the lance).
What, then, makes the lance different from a longspear? Something fundamental to its construction or something related to how it is used? If you use a lance overhead in two hands, how is that any different than a longspear? What would be the reasoning of the game designers in stipulating that you get double damage for using a lance instead of a longspear in this case?
I'm not sure that 2-handed use of a lance when mounted even is possible - at least, not while still treating it as a lance. Which could be an argument for allowing 1.5x Str if you want to preserve the lance as the cavalier's favored weapon. Or, as a compromise, create a feat (given for free to the cavalier at level 2) that allows 1.5x Str on a lance used in 1 hand.
