Keep the Mini. It's still a good MP3 player and just as long as it works and you haven't use it up, then keep it. You don't need to waste money just because something "better" came out.
In my opinion, the Mini has it's advantages compared to the Nano.
-Stronger. Unlike the Nano, sitting on it or throwing it on the wall won't kill it.
-More storage per price. I think the Mini 6gb was $250 and the Nano 4gb is $250.
-You'll know it's there. The Nano is so light and puny, that my friend sometimes forgets that it's in his pocket!
-Snazzy colors. Glossy white and black is cool, but the Mini had more variety.
-Isn't a scratch magnet. Unless you drop the Mini, you'll barely get any scratches. The Nano on the other hand, is so prone to scratches, I remember seeing this news articles about some lawsuits against Apple for something of the sort.
-Is good for big and small hands. The Mini is sized smaller than the iPod, but bigger than the Nano which makes it a inbetween for most hands.