Coffee vs. Diet Soda....which is healthier?

I'm a caffeine junkie. I need my morning coffee, and I also need my afternoon coffee. Hell, if I have a team dinner after work, then I need a diet coke to get me through it. Since I was spending way too much time and energy staying juiced up, not to mention the cost, I decided to cut back.

So, for the past few months, I created and then refined a spreadsheet to track my caffeine intake. Most of the raw data came from "The Caffeine Database" at energyfiend.com....

This approach has worked well.... I've totally eliminated the evening soda, and switched to decaf green tea for the afternoon. But I'm still struggling to get rid of my morning fix. Which brings me to the following dilemma...which is better for me?

I've read about the nasty chemicals and brain tumors that diet sodas are known to cause....but they have way less caffeine than a small coffee from Starbucks (again, citing the energyfiend.com). So do I take the risk of ingesting these chemicals on a daily basis again, or do I start being that weird guy that orders 3/4 caf, and then 1/2 caff, and then 1/4 caff at Starbucks?