That’s a tricky question.
It’s a personal preference which you notice as early as when doing crank stalls. One side feels more comfortable to stall on. It should be the same for grinds. When you are first learning, none is easier than the other. A difference starts to appear when you move to rails (especially round ones), where usually it is a bit easier for front foot riders.
Then, when you want to be able to grind on the other side (usually referred to as your “blind” side), you have to re-learn everything from the beginning, since the stance is very different. But all in all, both have their pros and cons.
There are some interesting discussions concerning this that were brought up in this thread, started by today’s best grinder, Chris H. (:p) : Unicycle grinds - Pretty gooooood! Chris Huriwai