Can't decide. I love both names equally. I would use either, it would come down to what name I was chosing to go with it.
Beatrice has a softer, more feminine sound, imo, so it could pair easily with a name in need of softening.
Beatrix is slightly harsher, and would pair well with a more frilly name, to create better balance. So say
Josephine Beatrice and
Angelina Beatrix.