Congrats on getting in. Personally, I think work experience is probably the most important thing for finding a library job. UCLA offers a lot of opportunities for working in libraries while getting your degree through their internship program. Especially if your undergrad degree is in History, you will have a great foundation to work in history related libraries. Yes, a subject MA might give you an edge, but is it worth the extra student loans?
I also got into UCLA, by the way, but I have the opposite problem. I moved away from SoCal, so now I am looking at out of state tuition to UCLA. Given that library jobs pay rather modestly, is it worth the massive student loans?