You may think I'm not religious, and if you think that, you're right. But, that isn't why I choose happy over blessed.
"Have a blessed day," has, at best, an element of luck or wistfulness built in to it. At worst, it involves a payoff for my being holy, or sacred, or whatever one needs to do to be blessed. In other words, if I do "good" I could possibly be "blessed."
The other choice, "Be Happy," doesn't involve chance, outside powers, or fairy dust. It's all about me and how I choose to be. I can BE HAPPY, or I can be something other than than happy.
That's why I always choose HAPPY.