Depends on the reasoning. Bitcoin is the brand, the name, the future. It will always be a store of value, very limited supply and it tradable against all other coins on exchanges. So it will continue to grow in value.
Bitcoin Cash on the other hand, has faster transactions, lower fees and is more likely to be adopted for day to day use. As it is partnered officially with Bitcoin it will continue to grow in popularity.
"Bitcoin Cash has the advantage of lower transaction fees and faster transaction times. The more widely used Bitcoin has been testing a new technology, called Lightning Network, that, if deployed at scale, could significantly cut Bitcoin Cash’s advantage..."
https://goo.gl/YjvaeV