Without question the biggest potential return on your investment is going to come from MTTs. However, you're not going to see quick and sure successes in them like you are in a cash game.
That being said, I personally find cash games to be tedious and, even though I might double or triple my money in a single session, I get more satisfaction out of making it in the money in an MTT.
Cash games are really there for you to exploit and profit from weaker players, and are really a means to a much bigger end. Alternatively, I feel there's more equal footing for all the players in an MTT. Therefore, while there's still an element of chance, you can leverage more skills in an MTT environment in order to win.
In the end, it all comes down to what you're trying to achieve and where you feel most comfortable playing.