I really do not know why people even ask which to invest anyway. The main rule of investment in any form is to diversify so as to at least spread all the risk and no one will even want to put all their eggs in just a single basket anyway. I am holding both of these even though I am holding mostly in bitcoin, and I have always traded my alts at their peaks since most of the time, they always get so beaten than you can imagine once there is a pull effect from bitcoin.

Diversifying even suggest not to go for big percentage into one single opportunity, I am preferring to go with bitcoin with major shares as I am very much confident about the returns of bitcoins compared to ethereum or any other crypto.