In our country we don't select PM,We have to select Party.
Now Which Party should come to Power is to ask as if congress comes they have declared their candidate for PM is Mr.Manmohan Singh who is also not very young compare to Mr.Advani(I think he is also of 77 yrs age).
So if BJP comes to power who should be Candidate as PM.I don't see anyone who is more accepted than Mr.Advani at present.
The question in india democracy is not who is more competent but who is more acceptable by majority of the people of different states,Mr.Modi may be more competent but he has to do lot more to be acceptable to other parts of India also apart from Gujarat.