To be honest - as an independent app store developer, these are REALLY hard to maintain just because you need to balance the need for ongoing development w/ servicing existing customers. Most app price points don't allow for this, since many customers are outraged that you would have the gall to even charge for an app (everything should be free, right?).
We've done well at doing both - and trying to add ONLY subscription services for features that are useful to users who use the app month over month.