I think that an ideal investor should do some of both because:

1) Long term investment is suited in case of coins like BTC and ETH. They have survived and grown for years and thus one can assume similar trends for future.

2) But you cannot use the same approach in case of all alt  coins. There are some alt coins which where removed from exchanges as no one was trading in them. So if you pick a new coin and wait for 2-3 years, it might not even exist till then. Thus, short term investment is better in this case.