Actually, if you look at the research, diet sodas are much worse or at least just as bad for you as the reg. kind. Even if you are looking at it from only a weight loss perspective, there are many studies that show that something in the composition of diet sodas actually can cause you to gain weight.
It is much better to cut out carbonated drinks altogether, there are so many health issues with drinking any soda, diet or otherwise. I broke a serious addiction to them about 5 years ago, and after the initial adjusting period, I don't miss them at all.