As someone who has never done a paid upgrade because I started my life as an indie on iOS, could someone explain to me why it's better than a subscription? Are users just significantly more likely to buy an upgrade than to just subscribe?

I'm currently experimenting with subscriptions on iPhone and I think it at least has potential. To be fair, my conversions aren't where I'd like them to be yet, but I do think there is potential. Many of my competitors are also using subscriptions, and they seem to be doing pretty well.

EDIT: It also allows you to do trials, as long the free version is useful enough on it's own you can happily give out trials of all the premium features.