There are a number of potential reasons you might be seeing a different result on two different machines. I'd recommend checking that you're using exactly the same tool settings, colour and metallicity settings ( the slider beneath the colour picker should be in exactly the same place, the further to the right the more reflective / textured your media will be ) on both.
If the result is related to pressure, it's also possible that your input device works in a slightly different way between the two systems ( different tablet, different drivers due to different OS or different tablet software settings ).
You can also reset all of ArtRage's settings to default by holding down the spacebar as you start up. You'll know this has worked if ArtRage immediately attempts to check for updates on launch.
If the above doesn't help, feel free to email me a screenshot from each to email@example.com with information on which OS version is being used in both cases, and I can take a look at them for you here to see what I can see.
EDIT: Screen gamma differences between the Mac and PC systems is another likely cause. Try opening the a copy of the same painting file on the Mac and PC and see if you spot any differences. If you do, this will probably be the reason.