Sales of diet Pepsi are falling because half of them are buying Pepsi Max instead. Not sure how it differs from the diet option. They both taste equally bad to me.
Depends which one you mean- apparently there's a "Pepsi Max" (nee "Diet Pepsi Max") on the US market which has more caffeine than regular Diet Pepsi. The "Pepsi Max" sold in the UK since the early 90s is really just... Diet Pepsi marketed towards men instead of women.
About 15 years ago, I tasted some (UK market) Diet Pepsi and Pepsi Max side by side just out of curiosity- the difference was minor at best.
The reason for having the two was- I assumed- more to do with marketing. Diet Pepsi and diet drinks in general were marketed and perceived as "girl" drinks, which probably put off male consumers. Pepsi Max launched with (very) 90s male-oriented advertising. (*)
What surprises me is that Coca Cola took around 15 years to do the same marketing trick with Coke Zero. That- at least- has the excuse of being a clearly different product from Diet Coke. (It sucks because it follow's Diet Pepsi and Pepsi Max's nasty over-intense "sweetness" rather than Diet Coke's less intense but more unpleasantly "hollow" sensation (**)).
On the other hand, it means that some men (e.g. my boss) might still have a legitimate reason to buy Diet Coke instead of Coke Zero, which he doesn't like.
Personally, I think almost all diet soft drinks are horrible, except Sugar Free Irn Bru because it's one of the very few- if not the only one- that avoids both the above pitfalls if it's properly chilled. Plus, it's not over-sweet, it's got caffeine in it, and IT'S SCOTTISH! (^_^)
(*) What's weirder; the fact that with the wonder of the modern Internet YouTube (cough) I can find obscure twenty-year-old adverts in under a minute, or the fact that that advert still seems familiar to me after all that time. (I remembered the annoying "ooh" at the end even before watching it).
(**) You might recognise this as "my mouth thinks this is sweet, but some part of my reptilian brain knows damn well this is carbohydrate-free chemical-flavoured water and is refusing to give me any sense of satisfaction in drinking it").