Basically it comes down to this.
1. All idrive equipped cars come with navi. You can't get I drive and no navi in a 3 series, the same goes the other way around.
2. -07 and -08 navi is garbage, just save your money and forget the idrive / navi.
3. -09 and up cars have awesome navis so then it is worthwhile to get I-drive.
4. There are virtually no differences between '09 and up navis (at least none worth mentioning or the money) other than a few features added here and there such as BMW Apps.
5. Bmw apps is available in mid -11 and up models.