it all depends how much big your music collection is. if you only 1 gig of music, I would certainly trade it in for the mini. After all, you'll still have 3 gigs of memory left. personally, the mini's hard drive is way too small for me.
and, instead of a trade in, i would sell mine on ebay. i'm sure you can a lot more than $130 for a 15gig Ipod.