R9 280x and 7970 are effectively the same card. I own a few of each and they both can get to 750 kHash equally well with modest overclocking and some tweaking of cgminer settings.
One thing to note, there are now Rev 1 and Rev 2 R9 280x cards out there. I believe the Rev 1 was literally a rebadged 7970, whereas I notice the newer Rev 2 cards come in a little lower voltage by default and run a bit cooler while maintaining the same hash rate.
There are many detailed comparisons out on the web with much more information than I can give you, but for mining scrypt I would take whichever card you can find cheaper. I might give the nod to the newer Rev. 2 280x's though if price is about the same as it does seem you gain a bit of power efficiency without having to mess around with under-volting yourself, especially with the cards you need to flash the BIOS to play with the voltages.
One thing to note, there are now Rev 1 and Rev 2 R9 280x cards out there. I believe the Rev 1 was literally a rebadged 7970, whereas I notice the newer Rev 2 cards come in a little lower voltage by default and run a bit cooler while maintaining the same hash rate.
There are many detailed comparisons out on the web with much more information than I can give you, but for mining scrypt I would take whichever card you can find cheaper. I might give the nod to the newer Rev. 2 280x's though if price is about the same as it does seem you gain a bit of power efficiency without having to mess around with under-volting yourself, especially with the cards you need to flash the BIOS to play with the voltages.
I am getting 7970s cheaper than 280x and its about $50-70 cheaper .... so considering what you said it would be best to buy a 7970 for the same hashrates then ........ btw what 7970 model are you using ? I think Ill go with 7970
