An alternative: Carbonated water, often called sparkling mineral water, club soda or seltzer water (not tonic water, as this often contains sugar and ALWAYS contains quinine).
Fill a glass with carbonated water. Add splenda/stevia to taste. Add some lime and/or lemon juice to taste.
Voila! You have your own diet sprite alternative.
Please note that not all carbonated waters are equivalent.
* 115mg sodium (Canada Dry 12 oz. can)
* 0mg sodium (Perrier 10 oz. bottle)
If you want to get even better, you can make your own carbonated water. Here's how.
Also, there's no sodium in the stuff you make at home.
PS: I'm not sure if Diet Coke made with Splenda comes in decaf yet, but just for info, it only has 35mg sodium per 12 oz. can.