I would advice to try Android, because:
- it's more simple
- less restrictions like Blackberry api signing or BlackBerry Alliance
- there is more samples/articles/OS questions
- android market vs BB world app
- it will be easy to switch on BlackBerry or pure J2ME in the future (just in case you would like to port your app to other platforms, Android uses extended java, BlackBerry uses java microedition, both use Eclipse + plugin as IDE)
- devices are less expensive in general wrong by itself and as an argument
