i just bought one of each, one 40GB iPod and a green iPod Mini - i have to say that i quickly ran out of space on the mini, it took only a matter of days before i had to start choosing what i wanted to carry around if a were to use the mini. on the 40GB i have no worries at all.
pricewise the mini was kinda 2/3 of the iPod, but since i intended to use the 40GB as an method to do serious system-backups aswell, it has been well worth the extra cash. and of course, being able to carry your whole library all the time is an advantage i appreciate.
software-wise i think they are quite equal, not much of a difference. if any, really..
ceroc addict has a point about the size, but i already find the mini to big to be carried in any tight pocket(like my jeans or whatever). hence, i always leave it in my bag or the pocket of my jacket, where the difference between the two is not an issue. if i use the mini or the 40GB i don't feel the difference. see?
the mini has now been gently handed over to be cared for by my girlfried.. i would have regretted buying only the mini, as it wouldn't have done me much good.. i would say go for the original iPod, the extra space will be well worth it in many ways.
alex.