Long term=I don't venture out top 100 normally (unless a super promising project with rep) Ex ETH or BTC right now.
Short term=Anything that has value and that upcoming roadmap something is happening. Ex ETC right now.
Agree I think for a long term investment you should choose a coin that has proven potential and reliable. eg Bitcoin, Ethereum, Ripple, Litecoin. This coin has a huge community and users and it will certainly continue to grow that's why for the long term it will be quite safe. for the short term you can look for new coins or old coins of course that have good short-term projects, I recommend buying DNR.
Sometimes mate, long term investments can be better if you have a good buffer holdings but if you didn't have it you'll end up with regrets. Emotions can be so hard to dealt with and this will be your worst enemy in most certain cases, when the coins you have slowly increase its value in the market. Short terms is highly advantage when you bought an uprising coins price, then you can sell it while bitcoin price these days still promising and has a good signs to increase in the next few months to go.