Need job advice

I currently work in a very busy L&D. I had a less than ideal orientation and am now on my own. I recently interviewed at a different facility that has a completely different feel than my current job. I was offered the position, but haven't yet made my decision. I really would like to stay at my current hospital, but would love to move to the post partum unit. I have been at this hospital for several years during school and have been on my current floor for 8 months (3 months as a nurse the rest as a tech). I want to talk to my current mgr about the situation and don't know how to approach it. Should I tell her I had an interview with an offer, but would really like to continue to transfer to post partum and stay on at their facility? My sister thinks it sounds like I am giving them an ultimatum. I really don't think I can continue working on their L&D floor with the high stress level there coupled with a family situation I have going on right now. I worked post partum as a tech and loved it. Any advice???