Stalin was responsible for more deaths, over a greater period of time. I think that's what it has to boil down to. Both were monstrous, Stalin just did more damage. Also, if your able to look past both of their tendencies for orchestrating mass genocides, (and I could be mistaken) nazi Germany's economy may have actually prospered during hitlers mad shananagans, where as I'm not so sure Stalins Russia was so fortunate.
While Hitler was busy with an elitist mentality, Stalin just killed to ease his personal paranoia. We must look at the death toll of both and why it exists. Stalin's is a lot higher and this is because he was unspecific about his murder. He ordered his NKVD to find any people, charge them as Trotskyites and kill them to instill fear and disciple into his people. He also killed many of the party members in the show trials to send a message. Hitler killed with an idea that he genuinely believed, but Stalin just killed.
However, i believe that Stalin was 'worse' due to his use of terror and the purges he used to keep control over Russia with the support of the NKVD. Stalin's purges killed over 25,000 army officers and it is believed he even ordered for his friend (Kirov) to be assassinated. Do keep in mind Hitler was also very evil and was also responsible for mass murder.
Hitler and Stalin were both bad, that's not in dispute. The actions of both had lasting impact, however, Hitlers impact on the world was greater.
I need more words, but my 5yo just climbed into my lap, so its one hand typing for now. Makes it tough to make a point.
Compared to Hitler, Stalin may have killed more , but when you compare both of their ideologies, Hitler is worse. If Germany had won WW2 Hitler would have conquered the whole world and would have killed far more than Stalin. We would be typing in German right now if it weren't for their defeat.