both are different style of play. somewhat similar, but you're trying to reach different goals.
in tournaments your goal is to survive. and the way to do it so to knock out players and to build up your chip stack. the positive thing about tournament play is for a small buy in you can win a big reward. (depending on how many players)...you dont even have to take first to get a nice pay-off. the negative is its sometimes long and if you get hurt (chip-wise) early on you could find it hard to come back up in the end.
when it comes to cash games, its different. the money you can make doesnt come in one large pay-off, rather in pots that you take if you win. on certain nights you can make 10x what you bought and on other nights you could end up losing money. unlike a tournament, you dont have to wait till the end before you decided to cash out. also, you can buy in as many times as you want if you get broke. the down fall of cash games are: coming up isnt guarenteed and you could end up playing with more money then you intended.
another thing is: depending on your style of play..one style of the game may do far better for you the other. if you like to play a tight style i suggest playing in cash games. you can easily sit back and wait before you decided to play a hand. the worst that would happen is you'll lose a few bucks in the blinds whenever the button went around. if you're a tight player tournaments might not be the game for you. sitting back and waiting could cost you because of the blinds raising and aggressive players kicking you out of hands. if you like to play aggressive, tournament style might suit you better because your aggresive style means you are capable of surviving longer because you can steal blinds and take down pots. in cash games, this might work...but you could end up over-valuing a hand against a tight player and end up losing money. If you are able to shift gears and play tight > aggressive, then i dont see why you shouldnt be able to play both.
also, its a matter of opinion on which you prefer to play. i like cash games more because of my tight style of play and the fact that i have the option standing up and leaving when i feel is the right time.