Note that at the moment you really have only three valid options: Iphone, Android and BlackBerry. If you are not targeting US markets then BlackBerry is a non-option as well.
(Nokia has too many platforms and the devices are with bad UX, Plam is dying, Window Mobile will eventually catch-up but not soon)
If you have a business application your users are most likely using BlackBerry or Iphone (Android is still on the rise).
Developing for either is a matter of market share and you should really check with your users what they are using.
Note that if you are a web developer you should consider taking on with you someone that knows how to develop to those platforms (it is NOT trivial) or even consider writing an HTML5 web-app that would work on Android and IPhone.
