I recently had a conversation with one of the tax law professors at my school regarding to do in a similar position. I don't have a master's degree, but am eligible to sit for the CPA exam. She told me that if you are going to an accounting firm, the CPA will be something that will be helpful and potentially required at some point in your future. If you go into a law firm with the expectation of doing tax or transactional work, the time spent to obtain the CPA credential will not be worth the benefit.
I think there would be a difference if you obtained your CPA credential and worked for a few years as an accountant before attending law school, but just having the CPA title without any experience, from what I understood, would not be worth the time you would have put into it.
There are a lot of areas where an accounting background could be helpful. I recommend seeking out a professor at your school that knows something about corporate legal practice (has experience, not just an academic) to get their take on things.