Both choices are viable options but both choices come with risks. With an ICO you can really get lucky and have the price moon and go 10-50x or even more. On the flip side, it could tank and go nowhere and you lose everything or most of everything. You have to be ok with either outcome, though. Pick projects you believe in and projects that are backed by great names in the crypto world. Even with all of that you can still lose, though.
By purely investing in BTC alone, you run the risk of missing out on all of the crazy altcoin gains that can happen. BTC is more stable, but you may regret being only in that as well.
The real solution is to do both.