That would depend on the concentration and exact form of aspartame in bananas vs diet soda. Perhaps the aspartame in bananas is less absorbed than with sodas. I looked, but I didn’t find much on bananas and aspartame.
The banana serves as an excellent example for why you shouldn't believe everything you read on the internet when it comes to aspartame. Even so, there are people who will continue to argue that consuming a banana is dangerous to your health. Silly, isn't it? If you eat a 4 oz. piece of cooked chicken with a glass of red wine, apple juice, or tomato juice, you will get about ten times more phenylalanine and aspartic acid than the amount found in a Diet Coke, and two to three times more methanol. Do you really think a small piece of cooked chicken and a glass of apple juice is toxic?
The fear of aspartame is irrational.