Without knowing what your app is, it isn't easy to say which revenue stream would be better suited for you. However, saying that WhatsApp operate in a sector which has several free rivals and is a paid app, and is successful so it doesn't really matter what type of app you have because, if it is a good app then people are prepared to pay for it.

Nevertheless, have you considered offering "2 versions" of your app?

As you could offer a "Lite" Version which has 1-2 features missing and Ads (including offering to upgrade to the "Paid" version) and a "Paid" Version which has the 1-2 missing without Ads. What is important however, is that the 1-2 features that you remove are essentially bonuses and not core to the operation of the app but rather a nice feature/addition to the app.

With regards to the user experience, just ensure the ads are incorporated into the design/experience of the app when you are creating it rather than making it feel as if they were just added at the end in order to make revenue from the application as this will reduce the affect upon the user experience from a users point of view - Although don't make the ads so integrated that they don't annoy users to pay to remove them (i.e. from your "paid" app) - they still need to be slightly annoying as you will get people paying for your app just so they no longer see the ads.