According to the McDonald's nutrition facts, here's how they differ:
A small Coke has 150 calories, whereas a small Diet Coke has 0. Both have 0 calories from fat.
A small Diet Coke has 30 mg of sodium, whereas a small Coke has 15.
A small Coke has 40 (I assume grams) of carbohydrates, whereas a small Diet Coke has 0.
I'm going to assume by one Coke a day, you mean 12 oz. Given the calorie difference, it would make a difference. However, you could cut something else out if you would rather still drink Coke. Or you wouldn't have to cut anything if you are within your desired calorie range.
If you mean one 2 liter bottle, yes, it will make a huge difference.
Of course, if you like other drinks like water, 1% milk, or OJ, they are far more beneficial to your body.
[This message has been edited by Shane (edited 05-17-2001).]