Duh - if you're thin why would you drink diet soda? Very few diet sodas taste anywhere near as good as regular. If you're fat (like me) you probably have other calorie intake issues regardless of what type of soda you drink (guilty as charged). If you drink nothing but water but still eat 5000 calories a day you will gain weight. If you drink 1000 calories worth of soda but only eat 500 calories of food you will lose weight. It's called conservation of energy. It takes energy (calories) to make fat. You can't make it out of nothing. See Second Law of Thermodynamics.
Personally I'm a Diet Coke guy. Actually, I'm even more of a Coke Zero (or Cherry Coke Zero) guy, but my wife prefers Diet Coke, so I end up being a Diet Coke guy. I think Diet Pepsi is disgusting. If I'm at a restaurant and they have only have Diet Pepsi as their diet soda I usually get iced tea - but I do really enjoy Diet Mt. Dew. I really don't like regular Mt. Dew much (too syrupy for me), where I do like Coke.
But I have a couple buddies that prefer Pepsi, and they really like Pepsi Throwback, with the cane sugar vs. corn syrup.
Regardless of preference, for years the Twins had Pepsi as their official soft drink but it was unavailable in the Dome. I would assume that the actual concessions won't change either way, but they might have the Gatorade jugs on the side line and cover up any Coke signs visible from the field during the NFL games.