When filing an I-601 waiver for an overstay would it be safe to say that it MAY be easier to get approved than if you had a criminal conviction or is it the other way around?

ya, i would think so

Criminal conviction makes it more difficult although in some cases it does depend on the consulate. For example, in Ciudad Juarez a "simple" overstay with extreme hardship proven currently qualified for immediate approval (as part of their new pilot program). But if criminal charges are also part of the story, no immediate approval is available and the waiver goes into the backlog of 9-12 months for deeper analysis and review.

Like Kitkat stated, it depends on the Embassy- it seems harder to get a waiver approval for an overstay in Canada than some of the other embassy's.

i've asked the same question to several consulate and uscis staff. all off the record responses to me indicate that the USCIS has the attitude that if you were convicted of a crime, served your punishment, and haven't commited any additional crimes , then you have paid your debt to society and are reformed. however for overstays, uscis seems to consider the 3 or 10 year bar the punishment for the crime of overstaying your visa. therefore, when asking for a waiver of the 3 or 10 year bar due to an overstay, you are basically asking for a parole from your punishment.

boils down to this:

convictions: you are asking for the privilage of coming to the US even though you were convicted of a crime and served your punishment

overstays: you are asking for the privilage of coming to the US AND for them to shorten the length of your punishment

Seems very logical, they must have a different view at CDJ and they probably account for half the waivers. Just an overstay at CDJ does not seem much of a problem, there was a thread here recently that suggested the opposite applied to London.