I took a 16 day cruise on Pacific Princess once, back in the days when you got a soda package that ONLY included soda from the gun. The only diet choice was Diet Coke. But the bars ran out of Diet Coke 8 days into the cruise. They had to wait for a specific port to re-supply (they apparently couldn't re-supply at any old port) so it was another 4 days with no Diet Coke until we got there. And of course, there were no refunds on those 4 days of soda package, even though it was the only diet soda available. Ever since that cruise, I'm just grateful when they have any diet cola available.
This being said, my preferred drink of choice is also Coke Zero, and I like cans so that we can take them on shore with us in ports. So I'm glad that the current soda package includes cans. I'll cross my fingers for Coke Zero when I board Regal next week, but if it's only Diet Coke, I'll still be fine.