Your finished product will have better mouth-feel (more body) and it won't be as dry as with dextrose. It will all around be a better beer. If you're up for an experiment, get another can of the same kit and do one with dextrose and one with DME. I did this back when I started with a Muntons kit (English best bitter I believe) and the difference was pretty obvious, DME all the way.
EDIT: I don't have the exact number but I believe you want 20% more DME than dextrose. You might want to search to be sure.