I consider them both pretty evil. Consider this, Stalin's mass killings were mostly before WW II (100,000 a year), and mostly from his purges and 'collectivization in early 30's with the latest estimate of around 20-30 million dead. His genocide wasn't a full time occupation to his death, unlike Hitler, but that's where the comparison stops.
When comparing the two, both were evil, both put people in concentration camps, both committed genocide, both were dictators, both...well you get the picture. The one factor Hitler had over Stalin tho, Hitler was insane. Stalin was just angry and distrustful.
Other things to consider,Stalin killed more people, Hitler started WW II, Hitler killed the Jews and some people in his nation, but he started the war that resulted in many troops dying on both sides. Hitler may be considered the greater of both evils; tho Stalin killed more people it is said, Hitler's action ensured that those loses were as high as possible.
It is my conclusion that Hitler may be the 'evil-er" of them all, but comparing the them is ludicrous, now that i think about it, so just forget about it!