This isn't always the case though there can be times if Bitcoin gains value the same thing goes with alts but this doesn't happen most of the times. Also if Bitcoin loses value not all alts would go down or suffer the same path as Bitcoin. I remember the time where Bitcoin lost value and Bitcoin Cash started pumping then a lot of people and miners went crazy over the movement.

If I had to pick one I would go for Ether since OP mentioned that he'll be holding his investments until December of this year.