As the others have said, Coke Zero contains aspartame. However, it also contains acesulfame potassium which is another sweetener, so it may have a little less aspartame than Diet Coke. (The effect of multiple sweeteners is generally greater than an equivalent larger dose of one... or so I have... read. Maybe. In something somewhere. No promises.)

They seem to have gone back and forth, and more it varies by country.

My experience in Sweden was that first they with big fanfare changed Coke Light (what they call Diet coke outside of English-speaking countries) to Sucralose, then introduced Coke Zero, which used Sucralose and silently changed Coke Light back to Aspartame, and then at some point silently also changed Coke Zero to Aspartame.

There's a version of Diet Coke sweetened with Splenda, but Coke Zero is sweetened with aspartame just like regular Diet Coke. The Splenda Diet Coke isn't bad, but it does taste a little different than normal Diet Coke or Coke Zero. Look for the Diet Coke cans with a yellow label to find the Splenda version.

I thought so too but if you look at the ingredients they both use aspartame.