Source? Mole for mole, I'm sure methane is worse than CO2, but how much of the carbon mass in a landfill goes to methane?

I think (but don't have proof) that it still is better if none of it escapes as methane.

If you don't turn your waste into heat, presumably, you would dig up oil, refine it, transport it to where you need it and burn that oil. Chances are that produces more CO2 than the collection and burning of waste fairly close to where it is produced (transporting waste from Italy to Norway would probably be a different matter)

Also, the controlled burning of waste (typically at high temperatures) can break down molecules that, if dropped in a landfill, may make it into the atmosphere or ground water.

More arguments fore/against at http://en.wikipedia.org/wiki/Incineration#Debate. I find it telling that quite a few of the 'against' arguments compare incineration not with landfills, but with other technologies/solutions.

Methane is tens of times worse than CO2, so burning would probably have to release quite a lot of carbon to match methane.