The right business model for your web app depends on what your competitors or the people you are replacing are already doing. Inertia will cause people to evaluate your web app versus how they are already solving the problem. So if what you are replacing does monthly saas billing, then you too should to monthly saas billing. Of course, going free or super reduced price can cause people to rush over to your web app though they will do so with a grain of salt because they will have already been used to another system.