I'm a nooby when it comes to Graphics cards but upon reading on forums...
I've been tracking the HD7870 series to buy recently and noticed the core and Double D versions tend be to on sale for ($215 CAD for the core and $220 for the Double D after rebates)
do you mean double dissipation (double fan) and core (standard gpu core at 1000mhz) ??, then i suggest the double d, more overclock room you could even overclock it to a 7950 performance level. More fans better cooling more overclock room without crashing.
I bought the Core, and right after that, the Double D went on sale. To be honest, my fan is running pretty cool, DD is probably only going to give you less than 4 degrees extra cooling for when you overclock.