I am basically in the same situation you are in, except I basically already took option 1. I stayed an extra summer and fall semester at my undergraduate institution to complete a second thesis and pick up my third foreign language because I felt my history experience was lacking (I switched from an Electrical Engineering Computer Science major to History). Luckily I received a research fellowship and another academic scholarship, both of which funded my extra semester (I attend a public university and I had a great deal on a rent-controlled apartment so my expenses weren't very high to begin with). But even after the extra semester of research experience, additional time with my primary LOR writer, and an additional foreign language (important for international history applicants from what I understand), I can't gain admittance into a top-tier Ph.D. program. Admittedly I didn't earn another degree and I didn't stay the full year, but my feeling is that you've already built relationships with your professors at BC and there's not much you'd be gaining from option 1 other than personal satisfaction and practice at writing. I'm guessing you are probably already a superb writer, and I don't think you need another A.B. to prove that. My opinion is take the 2nd route if your only goal is to get a Ph.D. in history -- getting to know professors personally at a top-5 graduate program sounds like a better deal to me.
Edit: And I forgot to mention, it kind of sucks staying an extra semester. All of my friends were earning big salaries, moving out to nice lofts in the city, buying new cars, and playing golf after work, while I was stuck in the library wishing I was on the driving range with them. In hindsight my extra semester was still worth it (especially since I didn't pay for it), but I think I would've made a better use of my time by graduating with my friends and moving on to a Master degree program.