I just had a similar decision and I purchased a 4 GB Mini last week (eBay). I received it yesterday and am impressed. I've already got my music loaded, playlists recreated and have been using the FM Transmitter while driving. I paid $190 w shipping for a two month old Green model with the Griffin FM Transmitter and a neoprene case (you can find better and worse deals on eBay). I think as a previous poster noted, if you want a Mini, you should get one now before there are no more new or slightly used models available.
I did not narrow my choice down to an iPod Mini or Nano right away (considered IRiver H10/Creative Zen Micro/other iPod models). But Apple rules on usability.
When I was in the Apple store, it was hard to resist the $300 video iPod because it is so small. Still, I wanted my MP3 player for jogging (slowly) and the Mini seems perfect for that (despite the hd).
There are lots of Minis for sale on eBay at the moment. I think my main reason for not going with the Nano is my budget - I wanted to stay under $200 for at least a 4 GB model. I was also concerned about the Nano's size (can something be too small). The mini is the size of my cell phone and I have not put that through the laundry yet - but something the size of the Nano is more likely to end up there someday. Also, just like software, I find it best to stay away from new releases:-)
Its up to you to determine how much music you really need on the iPod (you can get the 6 GB Mini but only 4 GB max Nano)). And remember, its a music player! Using a 1.5" or 2" screen for viewing photos and videos is overrated in my opinion. My "free" cell phone takes/views photos and I never use that feature. View photos on your computer and if you want a video player, buy one of the 7", 8" or 10" portable DVD players. Anyway, thats my 2 cents:-)
Either way, you'll enjoy your choice. Just get a case for either one right away.
Peter