Clearly, Bitcoin is one of the best cryptocurrencies for investment, but at the same time, it is very volatile in nature. If you see the growth graph of Bitcoin, you will find that it has gone down as much time as it has gone up. Even though the overall growth is quite significant, you cannot be certain that the Bitcoin market will grow ever in the future.
ICOs, on the other hand, are the best option when you need a proper bitcoin alternative. First, they give you access to a new market of cryptocurrencies, one that goes beyond the bitcoin. Second, you can invest as much money in ICOs as you want and for as long you want.