All men are sinners. I would speculate that practising believers in all religions, on average, commit less "sin" than non-practising believers and non-believers. From my personal experience with people this definitely holds true. A study may prove either way.
There are many moral atheists. Moral doesn't necessarily mean religious. But it's certainly the case that one who is serious about practising religion is always being encouraged to behave morally, against their selfishness and desires, they're continually thinking about what they do (seems like a horrible life to an atheist), whereas non-practising and non-believing people will find it much easier to commit a "sin" as other than their conscience, nothing else is really holding them back.