And people who like chocolate are much happier than people who like vanilla

Happiness depends on many factors including personality type and personal circumstances. You cannot possibly take an entire group of either believers or non-believers and judge which group is happier based solely on whether they believe in god or not.

I think non Christians are probably more realistic, just on a daily basis. Being in denial may increase happiness, but overall it’s still not a good place to be, just a survival mechanism, (or not, for those they chose to persecute or feel don’t fit in ).
Overall I don’t think it’s a belief in a God or gods that would make a difference but your general outlook and demeanour.

probably the christians... they are probably learning what it is like to be free of religious social pressures. Now if they just give up christmas.... we will be free of rude angry shoppers who buy gifts they can't afford to impress their church members... then we will be free of the snippy christians at work when they are trying to work extra hours to pay for all those christmas gifts they have over spent. The rest of us are happy ever day free of that religious drama

Beats the daylights out of thinking your every move/ thought is being judged, harshly.