Neither are heroes. Thanos wanted to kill half of all life in the universe, apparently not taking into consideration that in doing so pretty much all individual societies would easily collapse into anarchy and eventual destruction.
Light wanted to rule over a perfect world filled with only those he has determined to be good and hard working. He started by killing criminals, then all those who tried to stop him. You could actually condone those actions as being the Greater Good; however, he was planning to kill all those who don't actively contribute to society once he was accepted as God. At that point Lind L. Tailor incidents would be all over the place, and being lazy would warrant the death penalty.
At the end of the day, Light wanted to create practically a utopia entirely dependent on Kira, but everyone would have to help it or die. Thanos's plan would be utterly catastrophic for everyone.
Therefore, Light is more of a hero but he's nowhere near as righteous as some would believe. He went too far, would murder so many innocents, for that to be the case.