With the Mini you can get more audio and SQ support(via 3rd party firmware), but no video. The Mini can be upgraded to give you 30% more battery out of a larger battery for only $20. Not to mention the Mini at this point and time can go up 32gb of space(swapping out the microhd drive to a cf drive), and even higher once 48gb, 64gb, and 128gb cards come out for the consumer. Plus, I have been told because of the mini's size it is more durable than the Nano.
All you have to decide is do you want/need the video and photo support of the Nano.