If I have to choose I would pick Bitcoin right away because it's less volatile than alts imo. Medium term ranges from 1-3 years according to google if you're going to invest on something for that long Bitcoin is a good option because halving is less than 2 years but Altcoins can also do better if you invest on the right one.
I prefer Bitcoin not Altcoins. Though it's depend on your investment term. Would you like to specify your term?
Is it Long term or Short term?
If you read the first post you wouldn't ask this question.
The price of Altcoins is dependent on the price of Bitcoin.
Altcoins are dependent but not all follow the gains and losses of Bitcoin some tend to lose more and others gain more because they can be used for pumps and dumps.