I never said that diet cola was better than water. There's no doubt that water is best. Personally, I've started drinking more water at work, when riding and lifting, and even a little more at home. However, I just can't bring myself to drink water all the time. Its not really a sweets craving (as, aside from diet pepsi, I rarely eat or crave sweets. My cravings are more along the lines of Long John Silvers or Taco Bell, though I've been good for the last 20 days).
However, if the op is like me and just simply can't switch to water completely, diet cola IS a MAJOR improvement over regular in terms of calories. If a person drinks 2 cans a day, IIRC, that's something like 500 calories. That's a good sized meal right there. So if a person can get used to diet, that will help in a big way.