You should ask those faculty and/or employers (present or former) who know you best and can comment on your academic and personal qualities as they relate to the job or graduate school.
If you are certain of your career goal and plan to use the letter only for that goal, it should be written specifically in support of that goal. If you are unclear as to your future plans, you may ask for a general letter of reference to be kept in your file for potential future use. Keep in mind that a letter written for a specific purpose may not be useful in the future should your goals change.
Conventional wisdom has it that most graduate and professional schools greatly prefer confidential letters, those for which you have waived your right of access, because the writer may be more candid and, therefore, more helpful. Some faculty will write letters only when the student has waived right of inspection, so you should discuss this up front with the writers. However, it is your choice. FERPA further provides that once you have waived your right of access and the letter has been put on file, you may not reverse your preference.