Hello there,
I started on Coopers kits and still use them on many of my brews, as I do not have a big enough pot for a full 5 gallon boil.
I have mixed those ingredients a few times, and as far as the difference between using sugar, there is no comparison. Using sugar is a waste of your time, as I know I have personally dumped 2 batches that I used sugar as my primary fermentable. Please do yourself a favour and push the thought of using sugar as your primary fermentable FAR out of your mind.
I think this answers your question indirectly. May I also recommend that you boil (use 1.25QT water per pound of extract) your UNHOPPED extract for 60 mins (carefull for boil-over) with some bittering hops (an ounce would do). Add more bittering hops(another ounce) at 30 mins into the boil, and turn the heat off after 60 minutes total. Then, add some more hops (as much as your want), and add your COOPERS kit and stir to dissolve. Now you may add a little extra sugars for extra Alc % and/or flavour, I personally like to add about 100-300G honey or molasses to every brew. Using the above suggestions will give you much more quality tasting beer that your friends will love. No one liked my beer until I started doing the above. Keep in mind, you may adjust the amount of hops all you want, and you can experiement with the amount of UNHOPPED malts you want... I like to add more than coopers kit recommends.
I have also found that using different yeasts is a HUUUGE component to a very good tasting beer, you may want to try a strain other than the one that comes with your kit. I am lucky; I have a microbrewery 5 mins from my house who sells 3oz fresh yeast that they culture themselves.