Mynameismang :
What's really the difference between graphic cards brand? Why some brand are a bit cheaper than others? Have they different technologies?
The performance difference is very minor, perhaps 1% ~ 3%
However some have a backplate which adds a bit more to the price. A Backplate helps a bit with dissipating the heat and making it more sturdy and less bendy.
Some have better cooling on the VRAM, voltage regulators, etc which means you will be able to take the overclock higher.
Some have better cooling for the clock, more heatpipes or thicker heatpipes and more advanced cooling system to ensure the best temperatures which also helps when overclocking.
What GPU are you buying, i can suggest you one that is great for the price.