There are a few differences between tournaments and cash games.
In a tournament, when you lose your stack, you're done in that tournament. You don't have to make any decisions about when to quit, while in a cash game you can quit whenever you want-- you can make better quitting decisions than your opponents. Nobody decides when to quit in a tournament, they just quit when they lose or they win the tournament.
Another big difference is the stack depth. In a tournament, a 50bb stack is usually quite healthy. Often in cash games, especially of the live variety, people's stack depth can be well over 300bb. There's a lot more room to maneuver postflop with a deeper stack, whereas in smaller SPR situations that you often find in tournaments, the decisions are practically made for you by the time the flop is over.
One generally also has the ability to table select a little better in cash games, whereas tournaments assign you specific seats.
Overall, while there are nuances to both cash and tournament play, I believe that cash games offer more complicated decisions and therefore a chance to have a larger edge than in tournament play. I think cash games are harder.