Timestamps in MySQL generally used to track changes to records, and are updated every time the record is changed. If you want to store a specific value you should use a datetime field.
If you meant that you want to decide between using a UNIX timestamp or a native MySQL datetime field, go with the native format. You can do calculations within MySQL that way
("SELECT DATE_ADD(my_datetime, INTERVAL 1 DAY)") and it is simple to change the format of the value to a UNIX timestamp
("SELECT UNIX_TIMESTAMP(my_datetime)") when you query the record if you want to operate on it with PHP.
Timestamp (both PHP ones and MySQL’s ones) are stored using 32 bits (i.e. 4 bytes) integers ; which means they are limited to a date range that goes from 1970 to 2038.
DATETIME don’t have that limitation — but are stored using more bytes (8 bytes, if I’m not mistaken)
After, between storing timestamps as seen by PHP, or timestamps as seen by MySQL :
using PHP timestamps means manipulations are easier from PHP — see Date/Time Functions
using MySQL’s timestamps means manipulations are easier from MySQL — see 11.6. Date and Time Functions
And, for more informations between MySQL’s TIMESTAMP and DATETIME datatypes, see 10.3.1. The DATETIME, DATE, and TIMESTAMP Types