HD 7750 GDDR5 > R7 250 DDR5 > HD 7750 DDR3 > R7 250 DDR3
Best is if you find a HD 7750 with GDDR5. Sapphire is a lot better brand then PowerColor. I would propose to stick to brands like Asus, Sapphire and Gigabyte. XFX and EVGA are a mixed bag. Their high-end models are good, while their lower models are worse. Zotac, PowerColor, PNY, Palit, Gainward and etc. have lower build quality in general.