Hi. Since you asked which of the two choices is best, I would say in my opinion it would be the degree. The reason I say this is two-fold. First, most, if not all of what you will get in a diploma would be covered or subsumed in the longer degree program so doing both might be duplicative and expensive. Second, with the movement of several States to possible licensing of fitness professionals, you may be ahead of the curve by obtaining a degree in the field since this MAY become a requirement in the near future (I like to be proactive rather than reactive 🙂
I hope that this helps.