Personally I would go for a 100% BTC focused portfolio IF I had to choose between the two, but I believe that altcoins have higher growth potential than BTC just because we haven't seen any serious institutional funds pouring into them. Who knows what heights can them achieve if that would happen? BTC is the less risky choice though if the potential reward is somehow more conservative.
As you say, if you are a safe and low-risk person, invest 100% in bitcoin, and if you are high risk and big reward put your money in altcoin, depending on the benefit. The profit you want and the risk you can bear, in the altcoin market is often unstable so you need to watch closely and focus, for me, I will choose both, one for the long term and one for the short term.
.But first of all op you should never ask for financial advice here, there's a lot of people randomly giving thoughts around, take this discussion as a consideration but never pick it 100% .
Always making plan B if your investment went wrong, don't put your money in a single investment that's the fatal mistake made by newcomers and educate yourself to manage money properly, that's all the basic instruments that every investor/hodler should have.