I think AdWhirl is a good idea and I use it in ad supported applications. It allows you to use most of the major advertising companies and means that you can switch between different ones without creating new builds of your application.
Personally, I place iAd as the first monastery, since the rates are the best, but the fill rates are often quite low (and do not exist in many countries). AdMob second, and I used "home advertising" third.
The answer for you probably depends on where your users are and what exactly your application is doing. But that’s why AdWhirl is a good idea, you can change the odds as often as you want.
(One thing to consider if you expect tourists to use it: will they include data? You won’t get many clicks if your users aren’t connected to the network!)