OP, whats your career goal?
I studied law, but i did take several criminology modules at LL.M. level. It really depends on your career aspirations, interests and individual strengths. I found parts of criminology interesting, but an entire degree on it is a different matter. They are such different discliplines. It's worth just taking a glance at a textbook of each if you're not sure. Try Newburn particularly for Criminology, it's pretty accessible.