Both the fields are related but different in its approach. Architecture is related to designing, review and supervising the projects.
However B.Planning will provide knowledge about development of a city or town based on certain rules and regulations laid by the Govt.

It depends on your interest & capability but in both of them B.Architecture is a good option. It has immense growth and lucrative earning potential in abroad as well as in India. If you do it from a reputed institute then definitely you will get a good placement also.