We unfortunately haven’t written too much about church & religious advertising, but right off the cusp, I would say it depends on your advertising goals: Do you want a short message that can reach the largest number of people possible? Or a longer message that will reach a smaller, more dedicated audience?
A billboard will reach everyone, including those who belong to other churches/religious institutions and non-religious individuals. You’re limited to a short message — maybe 7 words tops, and a single, simple image.
Lastly, I recommend checking out these 2 pieces, advocating some of the pros and cons of billboard advertising for churches: