For me the answer is obvious. Bitcoin is a more reliable value. This is happening even now. In the long run it will be even more pronounced. The mechanism of inflation underlying the Ethereum will never allow him to become promising in the long term. He's probably already reached my limit.
Its true, that bitcoin is more promising than etherium. But etherium has also something to say and has good future in crypto system. I think its better if you'll be investing in both coins. Diversifying will help you not missed the profit coin has to offer and also to decrease the possibility of losing money. We never know what coin may bring, it may suddenly pump in value which will give you better profit. So better invest in both coins.