Some of the advantages are that the mother may feel she has more control, as she will know when the baby is to be born, that the pregnancy will not run over the full term and, for many, that the anxiety about the approaching birth is less. Medical advantages can include lower risk of pre-birth infection,pelvic floor injury and some other complications.
Disadvantages include: A c-section is major surgery, which is never without risks. As well as possible side effects for the baby from the anasthetic, for the mother there is increased risk of heart problems, blood clots or post-partum infection, among others. Overall, women need longer in hospital after a c-section than after a natural birth. Due to the operation the mother also can't be in contact with her baby straight away, which is distressing for some.
Generally, the best advice for anyone thinking about an elective c-section would be to talk to their doctor or midwife, but you can get an overview here.