Hey Henning,
I would go for 'free with IAP'.
If you have a paid version of your app, you already unlock some features. Of course you could add some additional features with iAP' but - you won't be able to provide a free app afterwards.
You can't turn back from a paid version to a free version with IAP because you would lock some features for users who bought (and paid for) the app before.
Apple does not allow the submission of 'light-apps' anymore - so they actually recommend to publish a free app with IAP. Furthermore is is statistically proven that this pay-model pays out.
I would go this way (in general terms) but I did not follow the mailing least recently - so I don't know what kind of app you are actually want to publish ...
Cheers,
Chris