Obviousy, if you
have go have a soda fix, you're better off with
diet soda than you are with regular soda - one need only look at the sugar numbers in the "leaded" kind to see the difference.
Keep in mind that canned/bottled
diet sodas are sweetened with aspartame (NutraSweet), which is controversial in some circles. (Let's not go there.) Fountain
diet sodas, on the other hand, are often flavored with saccahrin.
The good news is that the sodium content of almost
all diet sodas has been reduced over the past few years, so that you rarely find a can of
diet soda that has more than 70 mg. (It used to be a lot more.) Diet Coke, Diet Pepsi, etc.,. have reduced their sodium content to 35 mg. per serving. There are also some
diet sodas (DietRite comes to mind) that are both sodium and caffeine free.
As far as the caffeine, that's a matte of personal preference. As you may remember, high-caffeine sodas like Jolt were introduced a few years ago, but they never took off in terms of major sales. The major brands (Coke, Pepsi, etc.) will give you an option.
When I was into losing weight, I totally dropped
all sodas - and it worked. Even though there may not be much nutritional value to
diet soda, my own take is that carbonated liquids will tend to bloat you more than good ol' water. And I'm sure we've all heard the stories about how Coke is used to remove grease marks and baked-on crap from cars, etc.
On the other hand, a cheeseburger with a glass of milk doesn't quite cut it, which is why I continue to be an avid
Diet Dr. Pepper fan. (Except when I'm in the South, when I wonder why the fine folks at Mr. Pibb never came out with a
diet version.)