Note first the ingredient lists:
Diet Coke: Carbonated water, caramel color, aspartame, phosphoric acid, potassium benzoate, natural flavors, citric acid, caffeine
Splenda-y: Carbonated water, caramel color, natural flavors, phosphoric acid, potassium benzoate, sucralose (splenda), acesulfame potassium, caffeine, citric acid
The Diet Coke with Splenda has a more definite vanilla scent, and is generally sweeter- I couldn't really taste the sour note that I like. I didn't get a filmy feel that Coke with high fructose corn syrup leaves. There is much less of a chemical taste to the sweetness than in regular Diet Coke. Strangely, I've come to enjoy that strange little frisson of "What is that? Why, you strange, Ridley Scott-style sweetener!?"