Criminology is more sociology/psychology, where forensic science will be a lab-based core science subject.
If you want to work in a forensic science lab/department (where jobs are few and far between and highly competitive) then the forensic science would stand you in better stead. However it still won't guarantee you a job as there simply aren't many available.
If you want a wider skill base which leaves you with more job options, then the combination with criminology might be better.
However both will give you a good set of basic lab skills, which would help with any kind of lab-based work. Many of the forensics undergrads from my year ended up in lab jobs in industries like cosmetics or food science. Never underestimate the usefulness of being able to work in a lab without blowing yourself up or killing other people.