Keeping both sounds more logical, to divide the risk but if I have to choose one, I would choose Bitcoin. Everybody talks/writes about the future of Bitcoin, that it will be 20-25,000 USD this year but there are few who try to predict about ETH.
I'm not so sure that Bitcoin have bigger chances for rising value during hodl. But it is good idea to divide the risk just in case one coin drops in value or fails.