ADF Mobile is used to build native applications that will run on iOS and Android devices, such as phones and tablets.
ADF Mobile Browser is a set of ADF Faces components optimized for rendering in mobile browsers. It supports iOS and Android as well as older mobile OSes and devices, most notably older Blackberry models.
If going to 11gR2 is not an option for you, you certainly can use Mobile Browser to build mobile web apps. However, users will not be able to use your application if they don't have an active network connection. For native on-device apps, which will run even without network connectivity, you need ADF Mobile.