DDR5 at the same speed as DDR3 has twice the bandwidth (DDR3 -> double pumped data transfers per clock, DDR5 -> quad pumped). When you can, always spring for a DDR5 version of the same video card. Your performance will be quite a bit higher.
DDR3 is cheaper, so a lot of cheap video cards will use DDR3 and more of it (2GB instead of 1GB) to entice the unknowing.