I find all the talk on Aspartame quite difficult to ignore. It's not so much of an issue for me as all I drink is water and green tea, but i've seen a few discussions and a study or 2, couple of videos and find it quite alarming. There's a video - i'll try and find it and post - that discusses how there's been a big increase in brain tumours since around 1984 and, it just so happens that Aspartame starting getting widely used at this time. I found that trend similar to the low fat fight and obesity, which for me is one of the most significant things that show low fat doesn't work well - the fact that it began in ~70's and, since this time, obesity (and carb consumption) has risen steadily.
However, my knowledge on these subjects aren't great so you could also say i'm open to manipulation since I don't really know any better. However, green tea vs Diet coke/coke zero/whatever, the answer seems obvious. As PN state, 1 billion chinese can't be wrong. If you're like me, you'll not really like Green Tea but after a while you can't live without it.
You should keep in mind correlation does not prove causation. Frequently these experiments don't have proper control and variable groups too. You should also be wary of anything that is frequently found in chain e-mail.