It could be both, depends on your specifications on how you put your venture with but definitely I would go to BTC hence this is the buy zone of bitcoin right now due to bad market sentiments. Some altcoins are worth it too, any top 10 on CMC is nice to buy and hold too.
Bitcoin will always be my choice no matter what altcoin may bring me , but of course if we can diversify then why need to choose one? i am for diversification as I am holding altcoins(with best potential of course) than pure bitcoin or pure altcoin , as remember never put your eggs in single basket.
I think that saying about diversification is good to follow for someone who prioritize tokens and some random altcoins because both of these are risky. By diversifying, it allows them to reduce their risks. For BTC. It's definitely fine if you are going to all in your funds in it. This coin has no issues as long as we know how to HODL strongly so that we can survive any bear markets and unexpected fall on the price due to some circumstances.
Another reason on why going solo for BTC is good is because some cryptos can only depend on the performance of BTC. Matic, Tron and BNB are top altcoins and just like Bitcoins. You can also HODL them for longer terms.