Well it depends, if your needs are fulfilled with the mini then honestly there is no use for the iMac. If what you desire is an all-in-one computer then go for the iMac and if you're ordering the fancy 17" one, the the graphics card will come in handy in all sorts of ways. Changing the RAM in the mini is a very difficult task BTW. As for refurbished I can't comment on that as Im completely ignorant in that matter but I've read lots of people pleased with them. Hope this helped.
-Victor