definitely the R7 250. are you buying them? because if you are, then getting a used Radeon HD 7770 7790, or 7850/7870 is the better option
No, I already have GT 640 DDR 5 Variant card one of my friends is interested in trading his card but his card is a ddr3 variant. So you sure R7 250 with ddr3 memory's gonna outperform my ddr5?