But I would go with Firm B. Think about your terminal values/goals-- where you are wanting to end up. Money means nothing at this age. (I'm in school at BYU as well) If I were in this situation I would go with Firm B as you are more likely to enjoy your experience, build on what you have done, and build a stronger reputation for yourself. Also, I strongly recommend against giving a verbal acceptance. As if you go back on it, regardless of it being a verbal acceptance the company will look at you as being unethical in re-negging on your offer. This represents poorly BYU, you, etc.

Firm A could turn out to be a great experience too and could shift the trajectory of what you end up doing.

Make it a matter of prayer. Can't go wrong with prayer. Sometimes both options are great and God is fine with either one you pick because both will be valuable.

At the end of the day, this is just an internship, you'll gain good experience either way, and am glad you have options. Just be careful about how you represent yourself, school, etc.