Easier on Mars, because you generally don't have to worry about strong winds. The gravity is lower, so it requires less thrust. For some rocket engines, this is actually difficult, because you have a limited throttle range; the Merlin engines have been designed for this.
Also in your favor on Mars, your landing pad isn't pitching up and down on waves. On the other hand, the ground is not necessarily a smooth, flat, level pad. SpaceX has demonstrated the ability to hover, so as long as you have decent fuel reserves, you should be able to spend some time searching for a good spot.
However, in the case of using this technology to land on Mars, there is a significant difference: you would be using it to land a rocket (first stage and all) on the planet after having done a long coast from Earth and a violent re-entry. That is definitely more difficult than returning a first stage to the ground after lift off.