Bitcoin is said to be one of its kind and the first one to come into effect over the blockchain. Later on coins like ethereum and rest of the coins came into effect. The difference is one is bitcoin and the other is altcoin which is technically alternate coin.

That’s just terminology difference. In the reality nature of work is different for the, and each coin is having its own purpose to serve the particular project.

Ethereum, for example, is used to create smart contracts over its blockchain and thus other alts work on it as the basis of project.

You can invest in both coins as they are the top invested and trusted coins.