I think you should invest both. Bitcoin investment is more stable but profit is not as great as Etherum because Etherum is much cheaper than Bitcoin, which makes Etherum easier to price than bitcoin. But investing in bitcoins is still a very safe channel.
Investing in both the coins simultaneously will result in lot of troubles for you. You will have to manage both the coins and keeping the record of both the coins at the same time is really a difficult thing to do. Bitcoin is really a good option as compared to that of investing in ETH is only good if you are going for either short term investment of your money or you are going for day trading. Otherwise, ETH is not a good choice.