1lb corn sugar in 1 gal water will provide 40 gravity points, or 1.040. DME will make 45 points per gal. 3/4 cup of corn sugar will generally give a nice volume of carbonation. Since DME has more points you would need to use a little less. I think DME might take longer also. I've never used it corn sugar works fine. You might want to check the recommended volumes of carbonation for the style of brew you are conditioning. I will recommend sticking to corn sugar, its cheaper, works just fine.