I think both of them are very good and safe coins. Bitcoins is always leading the charts from the value. But ethereum is second place, and it has potential to overtake in far future bitcoin. Ethereum is the altcoin with the most potential. It ahs already a running smart contract plattform, where a lot of projects are build. There are many other coins who try to copy these smart contract plattform, but no one of the is successfull now.