Which is better choice - Microcontroller with on-chip flash or external flash

Dear Experts,
I was wondering which is a better choice - choosing a
microcontroller with on-chip flash or interfacing external flash to a
microcontroller without flash. Please comment w.r.t the following:
a) Which option turns out more expensive for a given flash size (say
256K)
b) Is the accessing of on-chip flash faster than accessing externally
interfaced flash? If yes, why? Is not the on-chip flash also
interfaced to the same data and address lines as would an external
flash chip be interfaced?

Thanks in advance.

Ganesh