Here are my thoughts:
If favors are a custom in your part of the country, go with something edible.
Anything you will be able to afford to purchase in quantity for all guests will be something pretty cheap. A plastic rosary isn’t everyone’s idea of a nice wedding favor. If it’s a small wedding and you could get something nice for each guests, perhaps this would be different.
I have too many plastic rosaries hanging around this house as it is. Many Catholics will probably leave it behind if they, like me, have enough small religious articles around.
Not everyone that will attend your wedding will be a Catholic, so they will likely leave this favor behind.
Now, something like a personalized prayer card might be cool. The make those for funerals, I’m sure you could get one for your wedding. But, I’d include that at the ceremony, not as a reception favor.