While a one-time fee seems preferable from a customer-standpoint, I personally prefer annual subscription fees. It allows for a potentially lower initial cost, making it more accessible to potential customers, but--more importantly--provides an obvious recurring revenue model for the author, hopefully implying on-going development. Ideally you would retain access to the app if your annual subscription lapses, but could receive upgrades after reviving your subscription. Whenever there is a one-time fee for an "average" app (no offense intended), I always wonder how the author can afford to keep updating it in the future.
While a one-time fee seems preferable from a customer-standpoint, I personally prefer annual subscription fees. It allows for a potentially lower initial cost, making it more accessible to potential customers, but--more importantly--provides an obvious recurring revenue model for the author, hopefully implying on-going development. Ideally you would retain access to the app if your annual subscription lapses, but could receive upgrades after reviving your subscription. Whenever there is a one-time fee for an "average" app (no offense intended), I always wonder how the author can afford to keep updating it in the future.