I would say to split your investment, Bitcoin if you want a steady growth and Ethereum if you want a fast growth but risky.
Bitcoin is the most accepted cryptocurrency as of right now, there is no sign of any cryptocurrency taking over Bitcoin anytime soon.
Ethereum on the other hand is revolutionary blockchain technology which may be used for many of future use.
TLDR;
Bitcoin = steady growth, less risky
Ethereum = fast growth, more risky
It depends on the type of growth you're talking about. If you refer to the size of the blockchain, yeah, it's growing fast