What do you mean why would it take longer to train Leia? Luke was OBVIOUSLY the one showing power; Leia was showing virtually nothing at all. The entire focus of the story is that Luke is the one to get this job done. The Leia thing is just a twist. Obi-Wan knew what he was doing.
And Yerssot is right; involvment in the Rebellion has nothing to do with it at all.
Luke only showed power because ob1 got to him first, perhaps if Leia had got to see ob1 then it would have been a different matter, them being twins probably means the power divide is equal, Leia does show power in being able to sense Luke at the end esb, Yoda obviously thought that Leia had some power or he wouldn't have said about there being another and Luke even says that she can have the same power as him.
2 things:
1) she CAN'T see the ghost, otherwhise she would scream or something because her FATHER stood there and she didn't even looked!
2) Being able to 'hear' Luke at the end of ESB doesn't have anything to do with being able to use the Force, not even a little. The best example to explain this is in ANH were a stormtrooper gets tricked by Ob1, it is the same thing; you are transmitting something in their heads