Hitler was in power for 12 years and Stalin for 21 - trying to calculate how many people WOULD have been killed under Hitler had he been in power for another 10 years would be merely speculative and impossible to assess, given the unpredictable nature of warfare.
So it probably is not useful to judge who is 'worse' in terms how how many were murdered under their respective regimes - if Hitler had had another 10 years he might have killed twice as many people, or more. The fact that he was not given the opportunity does not make him any less evil.
Hitler murdered people because of their race, physical disabilities (mental and physical) and their political behaviour. Relatively speaking, Stalin murdered people indiscriminately, based on his paranoia. I don't think there is an objective answer as to which is 'worse' in this case. It could be argued that Hitler was worse because he followed a rigorously crude, barbaric and highly perverse ideology that led to the deaths of millions of people; it could be argued that Stalin was worse because he did not impose such a rigid ideology, but merely wanted to dispose of ANY 'anti-Soviet' elements.
Any murder is evil - to speak of it in terms of statistics, I believe, trivialises the huge suffering borne by each individual.
Final note: it is a contentious issue as to whether Hitler had any Jewish heritage. It is commonly asserted amongst historians now that there is no evidence to support this story, but that it was used as anti-Hitler propaganda.
Ian Kershaw's biography of Hitler