It's not really that one is better than another. Ohms are a measure of impedance/resistance, which is essentially resistance to the flow of current. If your Onkyo specifies a wattage at 6 Ohms, it means that it can push that many watts through a circuit (in this case the speaker) of 6 Ohms. So if it is used to power an 8 Ohm speaker, it will be facing more Ohms than it is rated at, and will be able to push less power through it. In general this is not problematic. If it is used with a 4 Ohm speaker though, it can push more power through the speaker which will cause it to draw more power and possibly overheat. If I'm not on target here I'm sure someone will correct me, but that is my understanding.
My guess about why your Kenwood stays cool though has nothing to do with this. I would conjecture that your Kenwood is simply not drawing as much power as your Onkyo to begin with. What are the two models? By the logic above, the Onkyo should have an easier time with the m60s than the Kenwood does.
