While I'm no expert on this, I'll give this a shot.
Choose which answer helps you more
While upvotes can be taken into consideration, personally, I feel that you should choose the answer that helped you the most. If you think that the downvoted answer is better, go ahead and BA it. Unless the answer gives false information, there should be no reason that you wouldn't BA it as long as it helped (it obviously wouldn't help if it were to give false information).
You don't know why an answer was downvoted unless it is apparent, which in that case you wouldn't really consider BAing it in the first place.
So, in conclusion, BA whichever answer you think is best, not the one that other people like better.
Hope this helps! :)