Tournaments at their core are a series of short stacked battles - those who know what ranges of cards they should be playing and how to pick up chips without massive hands are those who have the most success in the long-run. There is a lot of luck involved because there is less room to maneuver and the blinds get so high you have to get lucky to run deep. Volume is key, if you lose a tournament online you can always hop right into another one.
In my experience cash games are deep stacked battles where you fling money at each other and see who has the most stuck to them at the end of the night (lol). There is more play in cash games, since you can make moves and get folds from people, so this is usually the more skill-based of the two.
Very different skills are needed to be successful at each game, so most people specialize in one or the other (or are mediocre at both).
Good luck to you!