The decission is hard if I have to choose one of them only. ETH is doing great things in smart contracts, but bitcoin is much better performing in the exchange markets obviously. This is up to your choice.
I will also choose Ethereum if I have to choose one of them only. Ethereum has many interesting technology features than bitcoin. Ethereum platform runs on smart contracts which is a new technological revolution of crypto currency world. Smart technology of ethereum help it was applied wider than bitcoin. And investing in Ethereum at this time can give much profit than investing in bitcoin.