Long Term: Bitcoins
Short Term: Altcoins
Bitcoins is the first cryptocurrency, and that makes it the most trusted and used amongst all the other altcoins, so its the most safe to get hold of. However, since the price is so high right now, don't expect it to increase tenfold in the next few years as it would be a little unrealistic, but possible though.
Altcoins, or Initial Coin Offerings (ICO), can have potential if researched upon, which makes it more profitable, however, there are many scamcoins revolving about ICOs and altcoins, making it more risky than having Bitcoin.
It is up to you, but if you are new to BTC, I suggest getting some Bitcoin first.

I definitely agree, if we invest bitcoin carefully , we will get much benefits . And I think its really a safe investment, it make us have many chances to get through risk. In bitcoin, when I make a decision , I feel a bit more risky, awesome . In my thought , investing in Bitcoin or altcoin ,all brings you much benefits as long as you feel comfortable and willing to invest it .Especially, you need to be more careful and decisive . For me, if you wonder between investing bitcoin and altcoin, you should choose bitcoin which makes you feel more safer.