US/Pacific and PST8PDT both belong to the “other” timezones, which come with this warning:
Please do not use any of the timezones listed here (besides UTC), they only exist for backward compatible reasons.
They should all refer to the same timezone, compare:
http://www.travelmath.com/time-zone/PST8PDT
http://www.travelmath.com/time-zone/US/Pacific
http://www.travelmath.com/time-zone/America/Los_Angeles
As such, you should use America/Los_Angeles, which incidentally should also be a pretty “natural” and easy thing to select for users.