According to this report, Hackney’s crime rate is slightly higher at 129.6 crimes per 1000 people vs. Southwark at 126.7 per 1000.
Here’s some good advice
Another thing you might want to look at are the poverty indicators, which give an insight into the quality of life in each borough. Southwark seems to rate better than Hackney in this too (crime and poverty are often related). http://www.londonspovertyprofile.org.uk/key-facts/overview-of-london-boroughs/