let me clear this up a little bit more.
Idrive IS navigation. Its just bmw's "brand" of navi you could say. but iDrive is literally the navigation system as well as incorperating other control functions for the car --> climate control, communications control (bluetooth/sync), entertainment control (cd/mp3/satradio/am-fm), and navigation control.
I hope this also helped you with grasping the actual concept of what idrive is, rather than the differences.
Another note you may want to know as posted above, 09 and up navigation systems are the updated ones with the new GUI which is much easier to use. 06/07/08 3 series cars have a pretty laggy idrive system which requires the navigation DVD be in its seperate dvd drive at all times when accessing navigation data. 09 and up models have their own hard drive built in the car, and is a million times less laggy. infact, i have not really ever noticed lag in my 2011 m3's iDrive system.
my 2006 325xi has the worst navi ever, I was so enraged because it took me so long to really feel comfortable with it. If it takes me a few weeks to get used to it, then i can see how it would take some people months or some people could never like it. but ive gotten used to it.
Hope I have helped you and given you the answer you wanted. good luck with your purchase