It could be posible that ethereum will surpass bitcoin but i think not this year. Yes it is better to hold ethereum over bitcoin because eth is emerging in the market due to ico using ethereum as payments. But bitcoin is still the best when investing in cryptocurrency due to its volume and demand in the market.
That's right, it will be difficult for ethereum for this year. Moreover, bitcoin has a huge fanatic fan. Bitcoin is a cryptocurrency that has a special place as a pioneer of all altcoin. It will be difficult to shift.
I think both crypto currency are really promising for the investment of 2018 because they are potentially a profitable coin. Much greatly if you have a lot money to invest also to other currencies because this gonna be the best year of bitcoin and altcoins in a future days.