In my opinion, it's a terrible trade off and you're probably going to bite more than you can chew. Let me explain. For example, let's consider two apps A and B.
A charges a $50 one-off fee and B charges a $20 recurring fee. Sure, A will grow exponentially and you will appear to be instantly profitable, but it's a risk.
Think of this - You take $50 from the customer only ONCE this means, for the next two years, you need to pay for his support from your pocket. More the customers, more the support needed and more you will end up burning your pocket.
The problem with exponential growth is that it becomes static after a while and this is when things will become ugly. It will be a difficult situation because you can't switch to charging your customers monthly, because you've already charged them once. You will have very few options left then.
I can't imagine supporting a million users through a support system that I'm paying off monthly, but from an income that isn't monthly.
I think the best way would be to charge the customer annually like $99/yr or something like that..
This is a good read:
http://justinmares.com/why-real-businesses-dont-charge-5mont...
In this case, it seems like the OP isn't running Sendy as a startup. He is working on Sendy as a lifestyle business which is making more than enough money for the OP and maybe more than some of us who are working on our own startups.
EDIT: Actually the same psychological effects that push entrepreneurs to charge a recurring fee, push the customers to prefer a one-off fee.
I've been looking for a dead-simple RSS-to-email subscription service for one of the sites I work for. The only offering I've found (and dislike) is from MailChimp.
[1] http://forum.sendy.co/discussion/40/rss-to-campaign-segments...
