I wouldn't worry about it - diet soda contains a lot of other things besides the sweetener that make it not such a good choice. I do use Mio and similar additives to my water sometimes for a change, and I do count it towards my water consumption.
Half Fanatic #610, Marathon Maniac #7173, and officially Goofy!
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All our dreams can come true, if we have the courage to pursue them ~ Walt Disney