many people use Splenda with no problem and others can't. When it comes to the sweeteners, it's kind of gray. :-( I use Sweet n Low which is on protocol but some have used Truvia as well as Splenda.
Sonic is the best! I had an occasional (maybe twice) diet cherry limeade at sonic during my last P3 and didn't gain. I drank it early right at the start of happy hour (2) and drank extra water after. I'm paranoid. lol It turned out okay, though.
Sonic doesn't provide the ingredients for their food items... I would be hesitant to eat there. Nutritional info is one thing, but I would also like to know what I'm eating, too. I have to wonder how much food dye is in those drinks as they are rather brilliant in color.