The diet coke...It has aspartame, acid, and a lot of other things...If you break if it down aspartame contains like formeldahyde which they put into ciggarrettes, but nevermind that. Aspartame has horrible long term effects and can cause dementia, weight gain (when you start to drink too much coke, your body will read the diet coke as sugar and produce insulin even though there is none), and aspartame messes with your brain. Also the acid in coke takes out calcium that you have in your bones and teeth, and it's pretty bad on your teeth too. You know without food coloring coke is green (just some random fact.)
The tea probably has sugar, and honey isn't bad for you, the sugar is, but that's just some simple carbohydrates. Your body might make some insulin, but your way better off with the green tea and a little caffeine than from what diet coke can do to you. And you can burn the green tea off. Once you get aspartame in your brain it never leaves.
Why don't you drink some juice with no sugar added, some milk, or some water instead...even coffee might be better. edit:You're better off with the green tea then with the diet coke, even though it has high corn fructose syrup, i think sugar is better than acid, and aspartame.