A lot of factors are involved here: game experience, distance, psychological preparation, etc.
MTT is a very dispersive game. Cash games have an advantage, because the variance is less than in MTT. So if you want stability, then play cash games.
The question of time is very important. If you can afford to play only 2-4 hours a day, then cash games for you will be optimal. Well, if you have 5-10 hours of time every day, you can choose MTT.
Choose the game, that you like best and more suited to your style.