You should only inform admissions of what you actually intend to do. If you fully intend to accept the job offer and pursue a part-time M.S., I would let the department know that. If you're not ready to commit to that job and think you would prefer to work on a full-time M.S., I would say give it a wait. I got my decision Feb 20th last year, so you should be finding out very soon. Either way, best of luck!
(Current Stanford EE MS, PhD hopeful)