It's easier to know how you're doing when you charge money. If you have 10 paying customers you're not doing well. If you have 100k free users you probably won'

It's easier to know how you're doing when you charge money. If you have 10 paying customers you're not doing well. If you have 100k free users you probably won't know how you're doing, you may need them to some specific behavior or need larger numbers to make your thing work. I wouldn't pick one because you think it will be easier than the other. It's safer to assume they'll both be really hard, but probably in different ways.