Actually, the return which these two will give in future depends on a number of factors. For instance, there is news that Google is going to ban crypto related ads and this will affect the prices of both bitcoins and ethereum negatively. If there are improvements in ethereum scaling issues, it may even take over bitcoins. If it is able to do so, you will see dramatic increase in the price. Thus my advice is even if you go with bitcoins, invest at least some money in ethereum as well for diversification.