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 agree that bitcoin is more stable, but it is important to understand the purpose of investment.
If, for example, a person has a desire to save his funds, bitcoin fits more, if we are talking about multiplying the invested funds in a relatively short period of time, then it is better to choose alts.
But in any case, before buying, everyone should study what will be buying, that would not buy shit or not buy at the peak prices.
But in general ideally distribute risks and invest in different coins.