I might just buy the old one...
I am ordering a new 27" iMac, the lack of Firewire in the existing one is a downer for me as I have some External FW HDDs that i use for my Media files (not much heavy lifting there...) My thinking is I want the extra screen-real estate so that I can improve my workflow when I am working my organization's website that I help manage... excel and word files being cut and pasted into the websites databases and vice-versa...
It occurs to me that I will have all the USB3.0 ports I need on the iMac, but no Firewire... My guess is that the new ATD will have the same ports as the iMac meaning no firewire... and hell, if I have an extra grand after the new one is out, my iMac becomes a 3X27" display monster!!!!!
my Point is if Firewire is of any need for your machine you may consider the current model, and not wait for the next one... then again if it has dual TB ports you could get the TB to FW adaptor...