help me out with compare

Solution
Pretty much yeah.

Just for a comparison, as said ealrier in this thread: the HD 7770 was rebranded as a R7-260x. There is almost zero performance difference between the two.
They are the same card, just different brands.

They will perform equally.

Either way you will need a quality 450 watt power supply with an 80+ rating.
Just because a company slaps a 450 watt sitcker on it does not mean that it is actually up to the job of supplying it. Never go cheap on the part that can destroy everything else.
 
If it is from different vendors then that could be the case. Some stores are going to try to charge near full price for old generation card, and others just want to get it out of their inventory without taking a loss so they price it differently.

If it is from the same vendor then maybee they got a good deal from gigabyte, or maybee the gigabyte is refurbished.