3 glasses of soft drinks is an insane amount of sugar on a daily basis, so if you have to do it you're probably better off with the aspartame.

One school of thought (including my own) is that if you’re consuming enough sweetened products per day that you need to consider switching to ones that contain artificial sweeteners (to avoid too much sugar), then it’s time to re-evaluate your lifestyle by questioning why you’re consuming so much sweet stuff to begin with. There are so many other foods that have so many other enjoyable flavors and well-known health benefits that you can make room for in your life.

I believe your options are glucose–fructose syrup or aspartame - I'd lean towards aspartame as well. If sugar was an option I'd gladly cut the daily intake and go with that.

Sugar has extremely well documented negative effects even in the amounts you find in a regular soda (let alone three). Most of the Aspartame studies test massive doses and are generally inconclusive. If it's a choice between the two, go with the Aspartame.