The hashrates of both cards should be similar to a great extent, it depends mostly on wether there is a great modified bios available for the card, and also depends greatly on your ram type, which you cannot find out before you open your card and test it sadly, I would get the sapphire if they're the same price if I were you, if it's more expensive then get the gigabyte.
in my country it almost 100$ difference
I dont understand so much at the modification of the cards, is there any tutorial or something how to modify the cards?
From what I looked there almost no one uses this cards and I cant get information on this card.
and how I can know if I can modify the bios?
Thanks for the help!