First of all investing to somethings doesn't mean that you will earn a profit, there's a high risk you're going to face whenever you're going to invest to something. So the both strategy is profitable but there's always a risk present in trading, so invest only what you can afford to lose. And it's not guaranteed that this strategy will work always, It depends also on the coin in where you're going to invest so you really need to study the coin first or analyze it before start trading.

LONG TERM INVESTMENT
Where in you're going to hold your coin for a long time and you'll withdraw it once you've reach your goal time/price.
High Profit
High Risk
Coins: Bitcoin, Ethereum

SHORT TERM INVESTMENT
Where in you're going to sell your coin once you've earn profit no matter how big or small it is.
Low Profit
Low risk
Coins: Ripple, Litecoin, Dash and other altcoins